The screenshot doesn't show the issue well. Does Thunderbird indicate there is an attachment and provide the link to download it? More info may be needed, as I am unsure what is happening.

There doesn't appear to be an attachment to the email. I believe what happens is that the attachment appears as 'text' in the email.

This doesn't happen always happen with all attachments. PNGs seem fine but DOCX have the problem. I believe that sometimes rebooting the computer solves the problem but this didn't happen on the most recent occasion. I tried starting Thunderbird in 'safe' mode but that didn't help.

Does this come from everyone , or just one person? There may be corruption in the sending. To verify the problem, try sending a DOCX to yourself to see if it comes in properly. I am aware of no bug in this, as that is a common practice.