I always recommend restarting the operating system in safe mode with networking for diagnosing issues like this.

Despite claims to the contrary, AV products generally only tell you they are off and therefore the problem they cause is not cleared (in one case it left an injected library running in Thunderbird's process). Safe mode with networking prevents them loading so you can be sure.

Thanks for your reply Matt. Unfortunately I have the same problem in Safe mode. I also tried Thunderbird Troubleshoot but to no effect.

Open the message source (ctrl+U) and look for the attachment header. It will look something like this, except with your file name, but the layout is consistent.

Content-Type: application/pdf; name="TDPB_1_1695087051.pdf"
Content-Disposition: attachment; filename="TDPB_1_1695087051.pdf"
Content-Transfer-Encoding: base64

Is the content type set to application/text or something else?

Hi Matt, Here is the attachment header:

Content-Type: text/plain Content-Transfer-Encoding: 7bit

I should also point out that the email with the attachment is created in a Visual Basic program using Microsofts CDO.

I changed the extension to TXT and it works okay. Still not working with a DAT extension

This has now been solved with version 91.7.0