File Extension Lookup

File extension lookup

A file extension is the short suffix after the last dot in a filename, like .png or .docx. It hints at the file’s format and which application should open it, and it usually maps to a standard MIME content-type used on the web. This tool lets you search a curated database of common extensions and instantly see the format name, MIME type, category, and the programs that typically handle the file.

How it works

The lookup normalizes your input — stripping a leading dot and lowercasing — then matches it against the extension database. Each record carries a human-readable format name, a canonical MIME type (for example application/pdf), a category (image, document, archive, audio, code, and so on), and a list of common applications. You can also filter the whole list by category to browse families of formats. All matching happens locally.

Tips and notes

• Extensions are hints, not guarantees — a file can be renamed. For trustworthy detection, check the file’s binary signature (magic numbers) or its server-reported Content-Type.
• Generic extensions like .dat or .bin have no single format; the entry flags these as ambiguous.
• Web servers should send the listed MIME type so browsers render or download the file correctly — see the related Content-Type lookup for the reverse mapping.

Why the same extension can mean different things

Some extensions have been repurposed over time or are generic by design. A few notable cases:

• .dat — no single format. Used by countless programs to store binary or text data. The content depends entirely on the application that created it.
• .bin — raw binary, used by firmware files, disc images, and proprietary data formats. Context is everything.
• .log — plain text on most systems, but some log-management tools write proprietary binary formats.
• .tmp — temporary files that could be almost any format; safe to ignore or delete once the creating app is closed.

When you encounter an unfamiliar extension and the database returns multiple interpretations, the best next step is to open the file in a hex viewer and check the first few bytes — the “magic number” signature.

Common magic number signatures

File magic bytes are more reliable than extensions. Here are a few well-known ones:

If an extension lies, the magic number usually tells the truth. Knowing both the extension convention and the signature gives you full confidence in a file’s real format.