So it's confirmed, thank you. Of course it's just a text signature, but still quite an annoying prank.
It's much more annoying than you think: it could fool an antivirus program into actually
deleting the file containing the offending signature.
I've seen Exchange databases suddenly disappear due to an infected attachment in an email message... it would be no wonder at all to have the whole blockchain wiped from your hard disk due to a fake viral signature.
And while this is actually harmless, having to download it again could be a royal pain.