mailDoc function is broken in JS (no attachment in email)

[sraka1]

In https://github.com/chromium/pdfium/blob/70c2db3e99bf97e1afbcd22ca37ef3b5ed101a51/fxjs/cjs_document.cpp#L349 it seems that an empty span is provided to the JS_docmailForm function (which is triggered by mailDoc from the JS context).

This results in the email client opening without the attachment, which is not in line with the Adobe spec.