Generally attachments that essentially disappear between the providers web mail and Thunderbird are caused by antivirus programs. Either corrupting the email in transit with their "scanning" or more frequently in the building of the file in memory and in the system temp folder when you choose to open or save it. The issue comes about because a "file" is not sent via email. When it is attached to an email the original file is disassembled and the contents is encoded in a text format called MIME. Then the email is send as a long sequence of plain text, indecipherable for most folks, but still plain text. When you receive the email and you ask to view or save the "file" which is listed in the attachment pane. The very first thing is this mime encoding has to be reversed to turn the pseudo "file" back into a file. This process occurs largely in memory, and on windows systems in the system temp folder where the resulting file is written. On mac systems the resulting file is written where ever the settings say. The issue is that the antivirus programs are so slow that at times the file is written and simply not available for the next step so the content is essentially corrupted. Try in the first instance without an antivirus product.