Then I suggest you learn where the executables are kept. Or reinstall Libre office so it can restate the settings for your operating system. The problems is really nothing to do with Thunderbird and all to do with your operating system loosing the settings. You might learn something on a slackware forum.

Perhaps something like this. http://libre-software.net/change-the-default-application-linux-mint-ubuntu/

I have posted this on a Slackware forum, but no solution so far. I have already done what you suggest in your ubuntu link for KDE, and csv, doc, xls file are opened with LibreOffice when selected from the OS file manager, but they are not selected when trying to open an attachment from Thunderbird. I've been using Tbird and LibreOffice for well over a year and this has not happened before. Since the OS does open these files with the correct application, but Thunderbird now does not, it seems to me it must be related to Thunderbird. Where does tbird specify what application to use? It must not be using the OS configured default.

csv gives "You have chosen to open <filename> which is CSV document." I get the 'Open with' defaulting to Kate. KDE file manager opens this correctly.

xlsx gives the same "you have chosen" dialog, defaulting to Ark. KDE file manager opens this correctly.

xls opens correctly in tbird. In Edit > Preference > Attachments > Incoming, xls is set to "Use LibreOffice 5.4 Calc (default). The others, e.g. Microsoft have no settings and do not show on the list.

So, a first step: how do I add types to the Attachments > Incoming > Content Type list?

re: how do I add types to the Attachments > Incoming > Content Type list? You do it via the actual attachment in email when selecting to open it.

re: The others, e.g. Microsoft have no settings and do not show on the list. message . As thunderbird has no info on what to use, thats why you get the message option....This link needs to be opened with an application. Send to:, then there is a 'Choose' button to browse the system.

There should be an option : select 'Open with' There should be a drop down list to select program to use. Select the appropriate option in drop down list.

If program is not in the list then you need to point to the appropriate .exe file. After clicking on 'Choose' you need to locate the directory of the executable file for that program.

select 'do this automatically for files like this from now on' click on OK

Well, in over two years of using Thunderbird this loss of associations has never happened before. Something strange happened with the last update, but I suppose I'll never know what. Meanwhile, help me work through this bit-by-bit.

I double-click on .xls spreadsheet attachment in Thunderbird. I get the message, "You have chosen to open: filename.xls which is a binary file from mailbox:// Would you like to save this file?" My options are 'Cancel' and 'Save File'. I have no "Open with" selection.

What do I do?

Please post image(s ) showing everything in the following location. If the scroll bar is visible on the left, you may need to provide additional images to show everything in the list.

'Edit' > 'Preferences' > 'Attachments' > 'Incoming' tab

After days of experimentation and several 10-hour LibreOffice builds, I've solved the problem. It was with LibreOffice, as Matt suspected. Both the source and binary builds were failing for some reason and the binary build in particular was messing up the .desktop files containing the mime settings. See https://www.linuxquestions.org/questions/slackware-14/thunderbird-not-opening-attachments-4175615401/ for details. I finally solved the problem by installing Alien Bob's LibreOffice package , http://www.slackware.com/~alien/slackbuilds/libreoffice/pkg64/, instead of the SlackBuilds packages. That work. I'm back in action.

Thanks for the feedback.