There is no "default" it uses the last used folder next time you click attach. Or at least that is my experience.

However those that have security and provacy software that routinely deleted the session.json file have all sort of issues with setting being forgotten like is the reading pane is turned on. (F8) between sessions, or between then running their chosen software like ccleaner and some anti virus programs.

Thanks Matt. I have the same experience last used directory opens. Tried to find the last used directory in *.json files (no success) and in the Win Registry, did not find a clear relationship between directory and Thunderbird.

Conclusion so far: no editable default directory and place internally stored by Thunderbird unknown.

you will find it somewhere in the preferences in the config editor but it will be updated every time the dialog is used.

Things were changed Matt in v52. Find the Avanced with General tab-page, but it is different with the help example and I do not recognize where I'm looking for.

You are looking for the button on the right that says config editor.

In my install it is directly above the cancel button.

Found Matth, thanks.

In the config editor mail.compose.attach.dir

Conclusion: no editable field.

With ATTACH EXTRA TOOLS you can define a set of Favourite folders for attaching files.

Thanks sfhowes for the reference. I appreciate it.