Thunderbird opens files based on the media type that the attachment is encoded as.

The DWG file extension is utterly meaningless in de5termining the media type. I appear to be seeing more and more poorly conceived email appearing on this forum from mostly web properties that send malformed email by failing to correctly encode attachments.

So before we waste any time tring to work out what is wrong with Thunderbird handling, please open the message source (ctrl+U) and scroll down until you encounter the attachments description in the message body.

It will look something like the following which is a PDF attachment in an email in my inbox.

Content-Type: application/pdf;
name="Thunderbird_CommunityFeedbackSurvey_Executive Summary.pdf"
Content-Disposition: attachment;
filename*0="Thunderbird_CommunityFeedbackSurvey_Executive Summary.pdf"
Content-Transfer-Encoding: base64

My understanding from the IANA web site is the correct media type for DWG files is image/vnd.dwg https://www.iana.org/assignments/media-types/media-types.xhtml

The 1996 discussion proposing the format and the accepting it is here https://www.iana.org/assignments/media-types/image/vnd.dwg

What I am finding is we have a generation of developers writing mail applications that really do not know the first ting about email other that what they experience in Microsoft Outlook. They tend to encode the attachments to email is a random binary file. This works for mail clients that ignore the internet standards for the interchange of file data using mime encoding (Microsoft products mostly) but it does not work at all in standards compliant products.

Не появляется для отметки надпись - "выполнять для всех таких файлов автоматически".

The inscription “Run for all such files automatically” does not appear for the checkbox.