Note: it's easy to create test e-mails with attachments using mutt.

echo test | mutt -s "test" -a files... -- user@domain

I did some testing with the following names:

<>
><
""
"<abc>"
<"abc">
>abc<
">abc<"
>"abc"<

There are no doubt other awkward names one can devise; in particular it would be useful to generate similar names with non-ASCII characters in them to force the use of value triples.