Change app used to open attachments
I frequently exchange emails with attachmented files in a specific format that used the extension .DUCX. I have set the Windows 10 default app for files with this extension to the applicable value (DUCX.exe), and double-clicking on any .DUCX file in File Explorer correctly opens it in the right app.
However, if I try to open a Thunderbird attachment with a .DUCX extension it opens in the app defined for text documents in Tools/Options/Attachments. If I try defining the app for text documents as Use other... >> DUCX.exe then .DUCX attachments open correctly, but real text documents don't.
It appears therefore that Thunderbird puts documents with the extension .DUCX in the same category as general text documents.
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This gave me some ideas but didn't solve the problem.
- I created a new profile and checked that the Tools>Options>Attachments table was blank. I then opened an attachment with a .ducx suffix and Thunderbird prompted for the app to open the attachment, to which I entered DUCX.exe and it opened correctly.
- I then used File Explorer to find which configuration file had changed in my profile. I discovered that the relevant file is handlers.json. I opened this file in Wordpad and it indeed appeared to reference DUCX extensions and define the relevant application
- I copied handlers.json from my test profile to my working profile and opened the profile in Thunderbird. Ducx attachments opened correctly.
- I then tried to open a .txt attachment and Thunderbird prompted for the relevant application, with the option of selecting the Windows default app (Wordpad). I accepted this, then tried again to open a DUCX attachment and it opened in Wordpad.
I have attached a screenshot of the Options window that shows the configuration for DUCX, but this line is apparently ignored and Thunderbird instead treats DUCX files as txt files.
From your screenshot this looks like as it should be. Do you see the problem only in the existing profile, or in the new profile as well? I wouldn't bet that copying over the file to the existing profile will ultimately fix the problem there. You should use the GUI Options.
The same applies to both profiles. When I define an app for opening text files that app is also applied to DUCX files, even if I have already defined the required app
open the message source Ctrl+U and look for the duck file that is opening in notepad. does it have the mimetype application/ducx or does it have the mime type application/text.
Thunderbird opens files based on their mime types. The RFC for email uses mimetypes to identify file types, not file extensions. File extensions are used only when the mimetype offers nothing useful.
There is an openattachmentbyextension add-on that might do it for you. https://freeshell.de//~kaosmos/index-en.html#openattach
DUCX files are in XML format, which you may realise comprise plain English text. The mimetype is application/text. I tried installing the OPENATTACHMENTBYEXTENSION add-on but I can't get it to work. It has added itself to Tools >> Add-on Options. If I try to add a new line a dialog window opens prompting for Extension and Command but whatever I enter nothing happes when I click OK or Browse. Can you give any instructions or examples?