The default opening app can be defined in TB when there is no entry in Attachments options. See the attached picture that shows the window that appears when you right-click, Open, an attachment that has no entry in Attachments. Select Other... to choose the opening app (in this case, text files already have a default app set in Windows). You can do the same with a .ducx attachment (delete the existing entry in Attachments).

Note that in the example of text files, the box 'Do this automatically...' can be checked - which stores the entry in Attachments. Now, in some cases the box is greyed out, which means the setting can't be stored permanently. This can sometimes be overcome by receiving a message with an attachment that is 'properly' encoded (depends on the sender's email program or service). I don't have a .ducx file to test if this is the case with TB, but you can try it yourself.

This doesn't solve the problem. To try it yourself:

• Create a text file containing the following:

  echo Hello World
  pause

• Save the document as Test.bat
• Double-click the file to open it. As a batch file it will open in a command window
• Attach the file to an email that you send yourself.
• Double-click the attachment in the received email. It will open as a text document with the name Test.bat.txt.
• If you open Options>>Attachments and delete the action for Text Document then double-click the attachment, Thunderbird will allow you to choose the application. However, this is the application to be used for opening .TXT files!

Don't double-click the .bat attachment - right-click, Open, and choose the opening app by clicking Other... and Browse... to cmd.exe, even if the text editor is offered in the dialog. I realize this will make txt attachments open in cmd.exe, instead of the text editor, because TB identifies the .bat mime type as a 'text document'. I'll have to think about this one for a while, but it should work for file types that are unknown or ones for which TB sets the correct mime type.

There was an add-on for older versions that allowed setting opening apps for specific file types.

Right_Click >> Open has the same effect as double-clicking. it doesn't give the option to choose the app unless there is no app configured for .TXT files.

I tried using Options>>Attachments to delete the app associated with text files, then double-clicking on a .DUCX attachment and selecting the appropriate app to open it. However, the file passed to the app has the extension .DUCX.TXT and is hence not recognised by the app. If I tick Do this automatically for files like this from now on then the app is used for any future text or unknown attachments (e.g. the .BAT file described in my previous post).

The add-on accessed from the link given is not compatible with the current version of Thunderbird.

In the Troubleshooting section of this article, there are a number of suggestions for getting around the incorrect handling of certain attachment types, such as modifying the Content-type header by editing an eml file in a text editor. Note that in current versions of TB, mimeTypes.rdf is replaced by handlers.json.

In the end, you may find your current method of saving .ducx attachments outside of TB is the only remedy. I don't know if other email programs would work better.