Thank you for your prompt response. I have already done what you are suggesting here. Thunderbird is already set to use layout to open the file. unfortunately it is adding .txt to the end of the filename. so for example an attachment named part101.dxf becomes "part101.dxf.txt". As i stated above this does not happen with a .dwg file, which is opened just fine by the same program.

same result. it adds .txt to the .dxf file every time. When i have googled this in the past i see that i am not the first to experience this problem.

Open the message source. (ctrl+U)

Scroll down till you see the attachment in the message source. The following is a guide. It is an attachment from one of my emails. the dotted line with the MIME parts code makes locating attachments and other objects fairly easy in most emails.

--------------CA977EDDE95B32D81F934291
Content-Type: image/png;
name="TB view more information about this update.png"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="TB view more information about this update.png"

What is the content type shown on the attachment, as well as the file name. My guess is the sender is encoding the attachment as plain text. But I am just guessing at this point.

the file comes to me as a .dxf but when i open it the file name is changed by thunderbird to .dxf.txt

this has apparently been a problem for a long time. here is someone else's registry mod to address the issue.

http://forums.mozillazine.org/viewtopic.php?f=39&t=2424905

If that works I would like someone to explain why.

Thunderbird does not as I understand it refer to the Windows registry at all on determining what to do with mimetypes. (very difficult to do on Linux and OSX). That is all stored in the mimetype.rdf file in the profile folder. So do we have another player in the mix here.

hopefully there is an easier solution!