1,717 extensions · 2,557 formats
What is this file?
Type an extension — the part after the dot — for what the file is, what opens it, and how to convert it.
Most looked up
Browse all →.rarA .rar file is a compressed archive — a bundle of other files squeezed into one, a bit like a ZIP. You don't open it directly; you extract what's inside..datA .dat file is a generic data file.binA .bin file holds raw binary data.dllA .dll is a Dynamic Link Library — shared code that Windows programs load while they run.bakA .bak file is a backup copy made automatically by a program before it changed or saved something.torrentA .torrent file is a tiny pointer file.tmpA .tmp file is a temporary working file that a program created and was supposed to delete.iniAn .ini file is a plain-text settings file.nfoAn .nfo file is a plain-text information file — either a Windows System Information report, or an old-school ASCII-art "release info" text file.vcfA .vcf file is a vCard — a digital contact card holding names, phone numbers and email addresses.isoAn .iso file is a complete image of a disc — every file and the disc structure, in one file.heicA .heic file is a photo taken on an iPhone
Plain-English answers
Every extension page opens with what the file actually is — no registry jargon before the answer.
What is an extension?Open and convert steps
Which programs open it on Windows, macOS and Linux, plus the practical route to a more useful format.
PRONOM-backed records
Technical data comes from the UK National Archives file format registry — 2,557 formats, fully cited.
Got a whole filename?
Paste invoice_final_v3.xlsx into the checker and it pulls out the extension. Nothing is uploaded.