Is there an example of an email client that attaches a file to an email named signature.asc for signatures other than cryptographic signatures?
The .asc part signifies that it’s an ASCII-armored PGP signature, which I don’t believe is a standard outside of cryptographic signature files
EDIT: And I don’t use S/MIME, but the other one (smime.p7s) seems pretty specific to cryptographic signatures as well.
signatures which are often redundant
I agree that a typical signature is usually redundant, but cryptographic signatures are metadata specific to the message used for the purposes of authenticating the message’s author and providing cryptographic integrity of the message’s contents. Unlike a regular signature, a cryptographic signature is not redundant.