I'm aware I can use the service for obtaining evidences of the contents and delivery of my emails, or of just its contents. Is this valid for any file format I attach to an email?
Absolutely. A digital file is, regardless of its format, a chain of data from which a hash can be obtained. No matter what type of file you send for registration, we will obtain its unique hash and register it.
The only limitation about email contents is in regards to its total size: emails exceeding 50 Mb will be rejected, but feel free to send as many as you want below this size.