Binary files have signatures in the beginning of the file.
You can verify if you open file with HEX Editor.
Startingpoint might be here: https://en.wikipedia.org/wiki/List_of_file_signatures
I haven't seen a highly technical document that really dives into exactly how the file blocking engine works. There's some mention of it in the official documentation. I also found mention of it being based on the content/file type and not just on a file extention in this document:
It's easy to validate this functionality for yourself. Configure a "file blocking" profile with action=alert for all applications and all file types. Attach that to a security policy that permits a test machine to use FTP. Take a pdf file and change the extension to .exe (or duplicate that file numerous times and also rename it to .bat, .jpg, .doc, .torrent, etc.). Use FTP to transfer these files through the firewall. Finally, look at the data filtering log to see the results.
I took a copy of the PDF file linked above, duplicated it a few times, forged the extension on all but one of the samples, and then transferred it through the firewall using FTP. The first snip is the directory with the duplicated/renamed files (all same date and file size). The 2nd snip shows the firewall logging the forged filename while identifying the file type as actually being Adobe PDF.
Click Accept as Solution to acknowledge that the answer to your question has been provided.
The button appears next to the replies on topics you’ve started. The member who gave the solution and all future visitors to this topic will appreciate it!
These simple actions take just seconds of your time, but go a long way in showing appreciation for community members and the LIVEcommunity as a whole!
The LIVEcommunity thanks you for your participation!