How to Open a File with an Unknown Extension

To open a file whose extension you don't recognize, first confirm the extension from the filename or your system's file properties, then look it up in a file extension database to see which programs can open it. If you'd rather not install anything yet, an online file viewer can preview many formats directly in the browser. FILExt, a file extension reference that has helped more than 50 million users over 20 years, offers both a searchable database and a drag-and-drop online viewer for this purpose.

Step 1: Find the actual extension

The extension is the part of the filename after the last dot — for example, .aac, .avi, or .dat. Two things trip people up here:

• Hidden extensions. Windows hides known extensions by default, so a file may appear as report when it is really report.pdf. Turn on "file name extensions" in File Explorer's View menu to see the real name.
• No extension at all. Some files arrive without one. In that case, check the file's properties (right-click → Properties on Windows, or Get Info on macOS) for a file type hint, or inspect the file header.

Step 2: Look up the extension

Once you know the extension, search it in a file extension database. FILExt maintains a manually curated database covering "almost every file extension there is," organized both by search box and by an A–Z index, so you can browse entries like aac, mp4, rar, mkv, zip, and others.

A database entry typically tells you:

• What the format is and what created it
• Which software programs and apps can open, edit, or convert it
• Related extensions that may work as alternatives

This is the fastest path when you want to know which application to install rather than just previewing the file.

Step 3: Preview the file without installing software

If you only need to see what's inside — or you're not sure the file is what its extension claims — use an online file viewer. On FILExt you drag and drop a file onto the site, and it analyzes the file type and immediately previews it online. The viewer recognizes thousands of file types.

Two practical notes from the site itself:

• JavaScript must be enabled for the Online File Viewer to run.
• Ad blockers can interfere. FILExt asks users to disable AdBlock and re-upload the file if the preview doesn't appear.

Previewing is a good first move because it confirms the file is intact and correctly identified before you spend time installing an application.

Step 4: Choose and install the right application

Use the database entry to pick software that matches what you actually want to do:

If the entry lists multiple programs, prefer one you already have installed, or one from the format's original creator — those tend to handle the format most reliably.

Troubleshooting: why the file still won't open

• The extension is wrong or missing. A file renamed with the wrong extension will fail to open in the "correct" app. Preview it first to confirm what it really is.
• The file is corrupted or incomplete. If a download was interrupted, no application will open it. Re-download from the source.
• The application is installed but not associated. On Windows, right-click the file → Open with → Choose another app, and set the default.
• The format is proprietary or obsolete. Some entries point to discontinued software. In that case, look for a converter listed in the same database entry.
• You're on a phone. FILExt explicitly covers opening files on both computers and cell phones, so check the entry for mobile apps as well as desktop programs.

Why use a dedicated extension database

FILExt was created by software pioneer Tom Simondi at the turn of the millennium and, starting in 2001, Microsoft directed Windows users to it for unknown file formats. It is cited in computer manuals and referenced by outlets including the New York Times for file questions. For an unknown file, that history is the practical point: the database is manually maintained and broad enough that a lookup usually resolves both "what is this?" and "what opens it?" in one step.