The Attachment struct exists, but this makes it glaringly obvious that
I've made a bad interface. The create_release_attachment should only
accept one file at a time and the loop over all files should happen in
main.rs
I've changed the signature, removed the loops, and wired in the newer
error handling routines. Needs fixing at call sites.
I've been putting this off because I don't want to have many small
formatting commits, and I don't have any kind of pre-commit hook to do
it for me. So here it is :v
