re :What I expect is that I can double click the attachment and some program will start and open the file,

It cannot open any file attachment until you have told it what to use. But once set up, then it will auto open using the chosen program.

Assuming you have not added information on what to use when opening eg: pdf document then you do this:

Right click on pdf attachment and choose 'Open' A small windows asks what to use ..'Open with', and usually a good option is already offered. In the case of pdf documents 'Adobe Acrobat Reader DC (default)' - obviously this must already be installed on your computer.

If you want to choose another suitable option, click on the drop down and click on 'Other' Another window opens and you can choose from an extended list of programs on your computer, but obviously you do need to select a program that can open pdf documents.

If a suitable program is installed but not showing, then click on 'Browse' and navigate to the exec file of chosen program and click on 'Open'

Select the checkbox 'Do this automatically for files like this from now on.' click on OK.

Thunderbird keeps a list of all attachment types you have selected to open and use automatically. This is where you locate that information.

• Menu app icon > Options > General
• scroll down to 'Files & Attachments' section

It will list the type of Content and the action used to open. You can also choose another Action as it offers a drop down of options.

Hello Toad,

thank you very much for your answer. I am using TB78.x on Ubuntu 20.04. May be it is important to know that I upgraded from the previous version 18.04. Ubuntu 20.04 was not installed on an empty disk. Since version 20.04 Ubuntu uses snap to install software. TB 78 is installed with the help of snap but the other programs are already installed earlier as .deb packages.

I assume that this is the reason why I cannot see those applications using the right click in TB to chose any other software option. Nothing else than TB appears. Afterwords I click on "other applications" and I find TB and Url Handler Script, that's all. TB presents the pdf icon in the bottom line where the attachments are listed.

TB 78 is not the official version of Ubuntu 20.04 right now and I hope they work on it because I usually copy the TB profile between Windows and Ubuntu to keep the mails up to date on all systems I use. The new profile does not match the old one.

Kind regards Ronald

I've just been doing a spot of reading up on this 'snap' you mention and I can't say I'm impressed. I prefer to have more control over what I want to install or not as the case may be.

Sounds like Thunderbird is running something like a sandbox and no access to other installed apps.

I did locate these links which you may be interested in: https://itsfoss.com/cant-install-deb-file-ubuntu/ https://linuxconfig.org/install-deb-file-on-ubuntu-20-04-focal-fossa-linux