Tell what a file really is MAGIC
An extension is just a label anyone can change; what a program reads is the signature inside the file. This reads that signature, names the real format, and tells you when the name on the file is lying.
The file
Drop a file here to read its signature, or
Drop any file. Only the first kilobyte is read, in the tab.
How it works
Why the name can lie
Renaming secret.exe to holiday.jpg does not change a single byte inside it: it still begins with MZ, the Windows executable signature, and Windows will still run it. The extension is a hint for the desktop, not a fact about the file. Every program that opens a file reads the signature instead, and so does MAGIC: it tells you what the bytes are, and when the name on the file disagrees with them.
Doing this often? The Sovereign Console loads whole sets of files as evidence, identifies and fingerprints each, and opens every one in the tool made for it. On its own, US$4 a month; with everything else, US$9 a month or US$79 a year. 7 days free.