I am pleased to advise that the Create Mail Merge function can now append attachments to generated emails!
In your spreadsheet, create a column with header of 'Attachments'. For each row, a list of file names can be provided as pipe-separated, and/or wildcards, and these files will be embedded as attachments at the top of the email body.
See screenshot for an example.
Before I release this enhancement, I want to work on a further enhancement to support in-line embedded images (MIME fun), but if that looks like it'll take too long I'll release this change as-is.