TrID-Net is a free tool - a GUI for TrID - which can identify more than 7,000 file formats from their contents alone.
The program could be handy if you've found a file that's lost its original extension, or maybe it has an extension you don't recognise and you'd like to know what it is, or where it's come from.
Please note: TrID-Net relies on a set of file definitions which aren't included in the download. To make it work, you must download the archive file from here and unpack it to the TrID-Net folder (a "defs" folder should appear in the same folder as TrIDNet.exe).
Once that's done, drag and drop your target file, or click Browse and navigate to it. TrID-Net then displays as many matching formats as it recognises, with the percentage probability of them being accurate.
For example, when we passed it a Word DOCX document, this was the result:
85.4% (.DOCX) Word Microsoft Office Open XML Format document (23500/1/4)
14.5% (.ZIP) ZIP compressed archive (4000/1)
DOCX files host Word documents in a ZIP container, and TrID-Net had correctly identified them both.