This is not a Thunderbird problem, but indicates the Windows shortcut to whatever program you use for these files has been deleted. Save the file and then rightclick the filename and select the proper file to open it. These are normally AUTOCAD files.

Thank you for your answer. We know, as I said, that we can save the file from email attachment and open it (for example - save on desktop, and open it). And that works. But earlier, we could open the file directly from email ( I will post a screenshot for better understanding, where you can see which "open" command doesn't work). Note - all other files (doc, pdf...) can be opened from email attachment.

Okay. Click Preferences> General and scroll down to the section on Files. You may have accidentally changed the setting for DWG, but should be able to add it back here. Let me know. Good luck.

Thank you, but I tried it, and that's not it :(

For other file types, Thunderbird relies on Windows for the program to open. You might check the default app for DWG files in Windows to confirm. That's my best guess. To confirm, switch to File Explorer, locate the DWG file, double-click to confirm that it opens the proper program. If it does, then Thunderbird may have a bug you can report. Good luck.