> I don't know how attachments are handled by other mailto-users though –

For instance, Icedove (Thunderbird) does not support attaches in
‘mailto:’ at all and this is considered a feature [0].

[0] https://bugzil.la/99055#c6

> multiple identical hfname's or some separator in the hfvalue like with
> the "to" header?

The former seems to be the only way if we want to use xdg-email(1),
since it requires that argument of ‘--attach’ should be existing file,
not an arbitrary string and in particular not a comma separated list of
files.

--8<---------------cut here---------------start------------->8---
$ xdg-email foo@example.org --attach hfsdg
xdg-email: file 'hfsdg' does not exist
--8<---------------cut here---------------end--------------->8---