Add file uploads

[wmnnd]

It should be possible to upload files of certain types (I'd say PDF, office documents for starters).

[gildenhard]

Would be super cool if one could send QR Codes, PDFs or at least .ics with transactional e-mails.

[marco-luigiano]

+1 — PDF upload is by far the most requested feature for me. Would eliminate a lot of workarounds.