FiletypeID is a compact portable tool which tries to identify the type of a file by examining its contents. And so even if you've found a mystery file which has lost its extension for some reason, FiletypeID may be able to tell you what it is.
The program is reasonably straightforward to use. Drag and drop a file onto it, click the "Analyze" button and that's it: within a second or two you should see its file type listed: executable, a PDF file, an image, MP3 or whatever (the program uses the popular TrIDLib library and so supports a lengthy list of formats).
There are still plenty of opportunities for confusion here, though, just because of the nature of what FiletypeID is trying to do.
If the program isn't entirely sure what type of file you have, for instance, it may display several possible matches (they're sorted in order of probability, but the language used can be technical and may not be that helpful to some).
And while FiletypeID is generally good at identifying file types, that doesn't always tell you their purpose. An example: when we gave the program a Word DOCX document, it told us this was a ZIP archive. This wasn't incorrect - DOCX files do use the ZIP format to hold their data - but plainly it can still be misleading.
Still, the program can provide useful information in many situations, and if you do ever need to identify a mystery file then it can be handy.