User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

Receiving an email that should have an attachment, but when reading the attachment is nowhere to be found. I chekced the email on Gmail via web and Android App and it is shown there normally.
The issue happened on version 60.9.0 (64 bits) but I also tested on the new version 68.1.0 and the bug is present there as well.

Actual results:

Checking the email as .eml format, I noticed it is there but it is not shown in the interface.

Expected results:

Showing the attachment as normal.

Here is a screenshot of the attachment header from the email on .eml format.

Please attach a minimal self-contained test case that does not contain private data.

As requested here is an example of the bug. This is a self made case from the original where I removed private information.
If this example is viewed on Thunderbird, the attachment won't appear, but if is uploaded to Gmail (using IMAP) the attachment will appear normally.
Hope this helps. Thanks.

Wrong message structure. It should be multipart/mixed not multipart/alternative.