ePrint: add support for non-PDF attachments

[alex-ter]

This adds general logic to support non-PDF attachment formats for ePrint, plus fixes a bug in case there are no attachments at all. I don't think there's anything beyond PS in addition to PDF, but it's easily extendable in the allowlist fashion if needed.

Relevant tests added (no attachments at all, just PS, both PDF and PS, PDF-only is already tested by existing tests). Tested using Scaffold in Z6/7.

Note that there seems to be a Scaffold-related issue where the text contents of Mathjax elements is no longer captured properly when the test is run, that affects some of the old and new tests. When running the doWeb() from Scaffold's Browser tab, it works as previously/expected, and when it's run from the Tests tab, it fails with delta indicating lack of Mathjax-contained text (mostly in abstracts). This is somewhat unexpected, so I'll report this as a separate issue against Scaffold shortly.