FortiWeb denies some uploaded files

[AEK]

Hi WAF admins

Sometimes my FortiWeb denies some uploaded files, just like pdf or png, and it logs an attack of type "generic attack" or "known exploit". The detected pattern can be something like this:

${�ǕN�������$�

Or something like that:

_/

I wonder if this is a real attack or just a false positive, since the signature is inside an uploaded file, while the string ${... looks like a kind of injection, and I think it should be blocked when it is in a form or in URL, not when it is in an uploaded binary data file.

Or maybe I'm misunderstanding something in WAF?