Micro Tools
Advertisement

Extension Checker

Check whether a file's extension matches its real format, free. Upload any file and this tool reads its magic bytes to detect the true type, catching renamed or disguised files.

Check a file

Drop any file below. The tool reads its signature bytes, never the full file.

Drop file here
or click to browse (any file type)
Advertisement

How it works

Every file format starts with a unique signature called magic bytes: PNG files begin with 89 50 4E 47, PDFs with %PDF, ZIPs with PK, and so on. This tool reads the first bytes of your file, identifies the real format from a signature database, and compares it with the extension in the file name. Only the first few bytes are read, so even huge files check instantly.

Why it matters

A renamed file can be harmless (a .jpeg saved as .png) or dangerous (an .exe renamed to .pdf to trick you into running it). This checker catches the mismatch before you open the file, which is a quick safety habit for any download you did not expect.

Related tools

JSON formatterFormat and validate JSON, privately. Regex testerTest regex live with highlighting. Base64 encoderBase64 encode and decode, plus URL encoder and JWT decoder.

Extension Checker FAQs

What are magic bytes?
The first few bytes of a file that identify its real format, regardless of the extension. For example, every PNG starts with the bytes 89 50 4E 47.
Can this detect viruses?
No. It only checks whether the extension matches the real format. A mismatch is a warning sign worth investigating, not a virus verdict.
Why does my .docx show as a ZIP archive?
That is correct. Modern Office files (.docx, .xlsx, .pptx) are ZIP archives containing XML, so a ZIP signature is expected.
What if the signature is not recognized?
The tool says so plainly. It knows the most common formats; an unknown signature just means the format is not in its database.
Is my file uploaded anywhere?
No. Only the first bytes are read, and everything happens in your browser.
Advertisement