I've been lobbying for the mimetype solution.

+1 ^{(populism != sanity)}

The bitcoin: URL scheme makes sense by analogy to mailto: but

Cool!

I have cooked up an email version,

One encrypted email from Bob to Alice requesting payment and attaching your file.
Another mail containing a copy of Alice's GPG keys so you can read the first mail.
Currently for the benefit of those happy to download and open an mbox file.
I suggest moving your ~/.gnupg/ out of the way while trying it, to avoid accumulating the cruft key.

Also may I suggest from my armchair,

HTH,

MrFlibble