About File-Extension.com

File-Extension.com is an independent, ad-supported publisher of practical guides on how to open, identify and safely handle file extensions — from everyday ones like .zip and .pdf to obscure or ambiguous ones like .dat and .tmp.

What makes this site different

Most "how to open a .xyz file" pages online are thin rewrites of each other. We take a different approach: for every extension we cover, we build a real, free, client-side tool that actually reads the file's bytes in your browser and shows you what's genuinely inside it — the same way a forensic or reverse-engineering tool would, just simplified for a general audience.

Concretely, that means our tools implement real parsers against the actual public specifications and reverse-engineered documentation for each format — the PE/COFF header layout for .dll files, the ZIP central directory for .zip/.xlsx/.apk, the OpenPGP packet format for .sig files, and so on. Where a format has no public specification at all (like .dwg), we say so plainly instead of pretending to parse something we can't — see, for example, our .dwg guide, which deliberately limits itself to what can be verified from the file's own version signature.

Every tool runs entirely in your browser. Nothing you open is uploaded to a server — we architected it this way specifically so you can safely inspect files you don't fully trust yet.

How we research and verify each guide

Every tool on this site goes through the same concrete process before it's published:

  1. Read the real specification first. We start from the format's official standard when one exists — an IETF RFC, an ISO/ECMA standard, a vendor's own published spec (Google's WebP container spec, AOMedia's AVIF spec, PKWARE's ZIP APPNOTE, BitTorrent's BEP 3, and so on). Each guide's "Technical references" section links the exact document we used, so you can go check it yourself instead of taking our word for it.
  2. Implement the parser from scratch, in the browser. We don't wrap an existing library or fake the output — the JavaScript reads the file's actual bytes (chunk headers, box structures, bit-packed fields) the same way the format's own spec defines them. That's a deliberate, harder-to-fake choice: it also means the file never has to leave your device to be inspected.
  3. Generate independent test fixtures and check against ground truth. Before a tool ships, we build real test files (not hand-typed fake data) with a separate, independent tool — usually a Python library, a reference implementation, or an OS-level utility — and compare our tool's output against that independent source byte-for-byte. For example, the TORRENT tool's SHA-1 info-hash output is checked against Python's own hashlib computing the same hash from scratch; the ISO tool's parsed volume label and file tree are checked against the Unix file command and a hand-built ISO 9660 image.
  4. Deliberately test failure cases. Truncated files, corrupted headers, files renamed to the wrong extension, and empty or malformed input all get their own test case, so a guide's troubleshooting section describes what a tool actually does when something's wrong — not a guess.
  5. Automate the check so it doesn't silently regress. Each tool has an automated browser test (using a real headless browser driving the actual page) that re-runs these checks; a tool only ships once that suite passes against real files, including at least one deliberately-broken one.
  6. Revisit and update. Guides are updated as formats evolve or as we improve a tool's accuracy or fix a bug we find later; the "Last updated" date at the bottom of each guide reflects the last time that happened, not just a cosmetic refresh.

None of this makes the tools perfect — formats have edge cases and real-world files are messy. If you find one that trips up a tool here, that's exactly what the contact address below is for.

How this site is funded

File-Extension.com is supported by advertising (Google AdSense). We don't accept payment to recommend a specific third-party program, and our tools favor showing you what's inside a file for free over pushing paid software. See our Privacy Policy for details on how ads and analytics work on this site.

Contact

Found an inaccuracy, a bug in one of the tools, or have a file extension you'd like us to cover? We'd like to hear about it: contact@file-extension.com.