Add test for multiple files with identical filenames

[DavidBiddle]

Currently the AttachmentList's []= method can be called multiple times with the same filename, and a new part will be added to the email for each of the files without overwriting previous files with the same name.

I've assumed that this is intended behaviour (since it's valid for an email to have multiple attachments with the same filename, or even have the same file attached multiple times).

This PR adds a fairly basic test to make that assumption explicit.