Missing MIMe headers with NewEmailFromReader #139
Description
In the latest master version (943e75f) when e-mails are created with NewEmailFromReader and they contain attachments, the attachments will have empty MIME headers. This is due to commit b84218f on Dec 30, 2020 which resolved a different issue. The MIME headers were never covered by a Unit test, so this was not picked up.
The old removed implementation however was also buggy as it set the Content-Id header of the attachment always to the file name. This is incorrect behavior as Content-Id can have a different value and should be read directly from the MIME header.