Find out what file type you're dealing with
Every file has an extension, the short set of letters after the last dot in its name, such as .csv, .dwg, or .epub. This extension is what tells Windows or macOS which app to use. If the extension is hidden, turn it on: in Windows File Explorer, open the View menu and enable File name extensions; on a Mac, select the file and press Command-I to open Get Info, which shows the file's kind. Once you know the extension, searching it directly (for example, "what opens a .dwg file") usually identifies the program it belongs to, which is the fastest way to find the right app rather than guessing.
How do I choose an app manually on Windows?
- Right-click (or press and hold on a touchscreen) the file that won't open.
- Select Open with, then Choose another app.
- Pick a compatible program from the list Windows shows. If nothing suitable appears, select More apps to see everything installed, or Look for an app in the Microsoft Store if you don't have one yet.
- Tick the Always use this app to open [extension] files checkbox if you want this to become the permanent default, then select OK.
You can also set defaults ahead of time from Settings > Apps > Default apps. Type the specific file extension into the search box there, select the result, and choose the app you want to handle that file type going forward, according to Microsoft's own support documentation.
How do I choose an app manually on a Mac?
- Select the file in Finder, then either choose File > Open With from the menu bar, or Control-click the file and choose Open With from the shortcut menu.
- Choose an app from the list that appears.
To make that choice permanent for every file of the same type, select the file and choose File > Get Info (or Command-I). In the Info window, click the arrow next to Open with, choose an app from the pop-up menu, then click Change All. Apple's own support guidance notes that clicking Change All applies the change to every file sharing that extension, not just the one you selected, so skip that button if you only want this one file to open differently.
What if no installed app can open the file at all?
If neither system offers any compatible program, you need to install one first. On Windows, open Microsoft Store from the Start menu, search for an app known to support that file type, and select Get or Install. On a Mac, open the App Store and search the same way, or, for specialised file formats not sold through either store, download the software directly from its developer's own official website. Once installed, repeat the "choose another app" steps above and the new program will now appear as an option.
Which unknown attachments should I be careful with?
Not every unrecognised file is safe to simply "figure out." Extensions like .exe, .scr, .bat, .vbs, and .js run code on your computer rather than just displaying content, and are a common way malware is delivered through email attachments and untrusted download links. If a file with one of these extensions arrives unexpectedly, especially from someone you don't recognise or in a message that feels off, don't open it. Confirm with the sender through a separate channel (a call or a new message, not a reply) before opening anything you weren't already expecting.
People also ask
What does it mean when Windows says no app found to open this file?
It means no installed app on the computer is registered to handle that file extension. You can choose an app manually from the list Windows offers, or install a suitable app first, then try again.
How do I find out what file type an unknown file is?
Check the letters after the final dot in the file name (the extension). Turn on file name extensions in File Explorer's View menu on Windows, or use Get Info (Command-I) on a Mac, then search that extension to identify the app it belongs to.
Should I open a file with an unfamiliar extension from an email attachment?
Only if you were expecting it and trust the sender. Be cautious with .exe, .scr, .bat, .vbs, or .js attachments, since these run code and are a common malware delivery method; confirm with the sender separately if in doubt.