By the end of this lesson you will be able to
- Explain how a file extension leads to a particular app opening a file.
- Change the default app for a type of file on Ubuntu and on Windows.
- Compare open-source and proprietary software and name the main licence models.
- Check whether an app will open or edit a file from another program.
- Judge the risk of a browser extension from its permissions and publisher.
Sooner or later a file opens in a program you did not expect, or an app asks you to pay when you thought it was free, or a pop-up offers to add a helpful extension to your browser. Behind each of these is a quiet decision the computer made, or a deal the software maker is offering you. This lesson explains how those decisions work so that you can change them, and how to read the deal before you accept it.
How the computer chooses an app for a file
When you double-click a file, the operating system looks at the end of its name, the file extension, for example .pdf or .docx. It then checks a list that says which app should open that kind of file. That list is called the set of file associations. The app it names is the default app for that type. Think of a post room: the label on the parcel decides which desk it goes to, and the clerk follows a list that says which desk handles which label.
- Step 1: Double-clickreport.pdf
- Step 2: Read extension.pdf
- Step 3: Check the listWhich app is default?
- Step 4: App opensDocument viewer
Read this figure as text
A flow showing double-click, reading the extension, checking the association list and starting the default app.
Four steps. Step 1: you double-click a file, for example report.pdf. Step 2: the system reads the extension, here .pdf. Step 3: it looks in its association list and finds the default app for .pdf, for example a document viewer. Step 4: it starts that app and gives it the file. If no app is listed, the system asks you which to use, or says that no application is installed for that file type.
The choice is not fixed. Installing a new program may offer to become the default for some types, and you can always change it. A default web browser, default mail app and default PDF viewer are the most common choices. Changing a default does not uninstall anything; it only changes which app opens first.
Changing the app that opens a file
Open a file with another app, and change the default
Menu names vary slightly between releases of Ubuntu and Windows.
On Ubuntu (Files)
Step 1: Open with another app once
In Files, right-click a file and choose Open With Other Application (or just Open With). Pick an app from the list and click Select.
You should see: The file opens in the app you chose, this time only.
Step 2: Open the file's Properties
Right-click the file and choose Properties, then open the Open With tab.
You should see: A list of apps appears, and the current default is selected.
Step 3: Set the default
Choose an app and click Set as default. Close the window.
You should see: Double-clicking files of that type now starts the app that you chose.
Step 4: Change web, mail and others
Open Settings and look for Apps (or Default Apps) to set the default web browser and similar choices.
You should see: A list lets you pick the default for web, mail, calendar and others.
On Windows 11 (File Explorer and Settings)
Step 1: Open with another app once
In File Explorer right-click a file, choose Open with, then Choose another app. Pick an app and click Just once.
You should see: The file opens in the app you chose, this time only.
Step 2: Make it the default
Repeat the steps but click Always instead of Just once. Or open Settings, choose Apps, then Default apps.
You should see: Files of that type now open in the app you picked.
Step 3: Change by file type
In Default apps, search for the extension, for example
.txt, and choose the app beside it.You should see: The extension shows the app you selected.
Step 4: Show extensions if hidden
In File Explorer choose View, then Show, then File name extensions.
You should see: File names now show their extensions, such as
notes.txt.
Choose an app to open notes.txt
Always use for this type of file: [ ]
- Marker 1: The file being opened
- Marker 2: Apps that can open it
- Marker 3: Always use this app: off for one time
- Marker 4: Select
Read this figure as text
A simulated dialog listing apps that can open a text file, with a checkbox to always use the chosen app and Select button.
A simulated dialog, not a real screenshot. Marker 1 is the title line saying which file is being opened, notes.txt. Marker 2 is the list of apps that can open it, with a text editor selected, a document viewer and a web browser. Marker 3 is a switch or box to always use the chosen app for this type of file; leave it off for a one-off choice. Marker 4 is the Select button.
When an app cannot open a file
A file format is an agreement about how information is stored. Some formats are open and any program may read them, for example .pdf, .odt, .png and .txt. Others belong to one company, for example some older document formats. A program that does not know the format may open the file badly, with the layout changed, or not at all. This is a compatibility problem. When you share a file, choose a common format. For a document that should not change, save a PDF. For something that others must edit, check that their app opens it correctly before relying on it.
Open-source and proprietary software
The Licence on a program is the legal permission that says what you may do with it. Open-source software publishes its source code, the instructions written by programmers, so anyone can read it, and usually copy, share and improve it within the terms of its licence. Ubuntu and the LibreOffice office suite are examples. Proprietary software keeps its source code private, and its owner controls how it may be used. The word free can mean two things: free of charge, or free to study and change. A program can be one without the other.
| Question | Open-source | Proprietary |
|---|---|---|
| Can anyone read the source code? | Yes | No, it is kept private |
| Is it always free of charge? | Often, but not always | Sometimes, but often paid |
| Who fixes problems? | The community and the maintainers | The company that owns it |
| Can you share copies? | Usually, under the licence terms | Only if the licence allows |
| Examples | Ubuntu, LibreOffice, Firefox | Windows, many commercial programs |
How software is paid for
Before you install, read how the app expects to be paid. The common models are listed below. None is wrong, but you should know which one you are accepting.
- Free of charge: no payment. Some are open-source, some are supported by adverts, some collect your data. Read what they ask for.
- One-off purchase: you pay once and keep that version, though later major versions may cost again.
- {{freemium|Freemium}}: the basic app is free, and extra features cost money. The price of the extras is shown after you start using it.
- {{subscription|Subscription}}: you pay every month or year. If you stop paying, you may lose access to features or to your files in that service.
- Free trial: full features for a limited time, then payment. Check whether it renews itself automatically, and write down the date it ends.
- No payment
- May show adverts
Check data use.
- Pay once
- Keep that version
Upgrades may cost.
- Basics free
- Extras cost money
Read what is locked.
- Pay regularly
- Stop paying, lose access
Watch renewal.
Read this figure as text
Four columns for free, one-off, freemium and subscription software with a warning for each.
Four columns. Free of charge: costs nothing now; watch for adverts and data collection. One-off purchase: pay once and keep the version; watch for paid upgrades. Freemium: basic features are free; the useful features cost extra, so read what is locked. Subscription: pay monthly or yearly; watch for automatic renewal and for losing access when you stop paying.
Browser extensions
A browser extension, also called an add-on, is a small program that lives inside your web browser and changes what it does: block adverts, check spelling, save passwords. They are useful, but an extension runs inside your browser, where you see your email, your bank and your messages. A careless or malicious one can read pages, change them or record what you type.
When you add an extension, the browser shows a list of permissions, the things the extension asks to be allowed to do. The most worrying one reads like “Read and change all your data on the websites you visit”. Some extensions truly need that, for example an adverts blocker. Others do not. A colour-picker should not need access to every page.
Add Colour Helper? Publisher: unknown, 12 users
It can: Read and change all your data on all websites
- Marker 1: Name, publisher and users
- Marker 2: Permission: far too broad
- Marker 3: Cancel is the safe choice
Read this figure as text
A simulated browser prompt asking to add an extension called Colour Helper that wants to read all website data.
An educational simulation, not a real prompt. The title is Add Colour Helper, a fictional extension. Marker 1 is the extension name and its publisher, here an unknown publisher with 12 users. Marker 2 is the permission line, which reads: Read and change all your data on all websites. That is far more access than a colour tool needs. Marker 3 is the Cancel button, the right choice here; the Add extension button next to it would accept the permission.
- Do you really need this extension?
- YesIs it from your browser's official extension page?
- YesDo the permissions match its job?
- YesDoes it have a named publisher, many users and recent updates?
- YesDo thisAdd it, then review your extensions every few months and remove the ones that you do not use.
- NoDo thisChoose a better-known extension that does the same job.
- NoDo thisExtensions from random websites or messages are a common route for malware.
- YesDoes it have a named publisher, many users and recent updates?
- NoDo thisExtensions from random websites or messages are a common route for malware.
- YesDo the permissions match its job?
- NoDo thisFewer extensions means less risk and a faster browser.
- YesIs it from your browser's official extension page?
Read this figure as text
A decision chart that checks need, source, permissions and publisher before adding a browser extension.
Question 1: do you really need it? If no, skip it. Question 2: did you get it from your browser's official extension page? If no, do not add it. Question 3: do the permissions match its job? If no, do not add it. Question 4: does it have a named publisher, many users and recent updates? If yes, it is reasonable to add; if no, choose a better known one. After adding, review your extensions every few months and remove those you do not use.
To review what you have, open the browser's menu and look for Extensions (in Chrome-style browsers, the puzzle-piece icon; in Firefox, Add-ons and themes). You can disable one temporarily, or remove it. If a browser suddenly shows adverts, a changed search engine or a new toolbar, an extension may be responsible.
Common mistakes
✖ Instead of: Renaming
photo.jpgtophoto.pngto make it open in another app.✔ Do this: Changing the extension does not convert the file. Use Open With, or convert it in an app.
✖ Instead of: Assuming that free means free of charge and free of conditions.
✔ Do this: Read the licence model: adverts, data collection, trials and renewals can all apply.
✖ Instead of: Adding every extension that a website suggests.
✔ Do this: Each extension can see what you do in the browser. Add only those you need, from the official page.
✖ Instead of: Uninstalling an app to stop it from opening files.
✔ Do this: Change the default app instead. It is quicker and you keep the app.
If something goes wrong
- The Open With list does not show the app I want.
- Check that the app is installed. On Ubuntu use Open With Other Application and look through the full list. On Windows choose Choose another app, then More apps.
- The default keeps changing back.
- An app may be resetting it when it updates. Check its settings for a box such as Always check if this is the default. Set the default again from Settings.
- A file will not open in any app.
- Check the extension. If the format is unusual, ask the sender to send a common one such as PDF, or search the maker's official site for a viewer.
- My browser has adverts, a new home page or a toolbar that I did not choose.
- Open the extensions page and remove anything that you do not recognise. Reset the browser settings if it continues, and scan with your antivirus.
Key terms
- Default application
- The program the computer uses automatically to open a certain kind of file or link.
- File association
- The computer's list that says which app opens each type of file, according to its extension.
- Proprietary software
- Software whose source code is kept private and whose use is controlled by its owner.
- Freemium
- A model where the basic app is free and extra features cost money.
- Subscription
- A payment every month or year for continued use of software or a service.
- Browser extension
- A small program inside your web browser that changes what the browser does.
Practise it for real
Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.
Change and restore a default app
Create a text file called notes.txt in your practice folder. Right-click it and open it once with a different app than the usual one. Then find the current default for text files, write it down, change the default to another app, double-click the file to check, and change it back to the original.
Challenge: audit your browser extensions and one licence
Open your browser's extensions page. For each extension, write down its name, its publisher and the permissions it has, and decide whether you still need it. Remove one that you do not use. Then pick one app that you have installed and find out, on its official page, which of the licence models in this lesson applies to it and what it costs after any trial.
Knowledge check
6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.
Summary
- The file extension and the association list decide which app opens a file.
- You can open a file once with another app, or change the default.
- Open-source software shows its source code; proprietary software keeps it private; free can mean free of charge or free to change.
- Check the payment model: free, one-off, freemium, subscription or trial.
- Browser extensions can see your browsing, so read their permissions and add only what you need.
Checklist: can you do these?
Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).
Related lessons
References and further reading
Official sources for details that can change between versions.
- Ubuntu Desktop Guide: Open files with other applications (Ubuntu)
- Change default apps in Windows (Microsoft)
- Chrome Web Store and extensions help (Google)
- Manage add-ons in Firefox (Mozilla)