As it appears to me, the new version of k9 no longer is compatible with file manager total commander v3.23. When marking a file in TC, selecting ‘send to’ it is calling and starting k9, but the selected file is missing in k9. This applies to all of my devices running on Android 7.1, 8.0 and 8.1. While using the previous version of k9 everything worked as expected.
Are there any suggestions what I can do - beside going back to k9 previous version?
Your advice will be highly appreciated!
Never used that feature. But I can reproduce it as well.
See Display info dialog when receiving Share Intent with file:// URI · Issue #4399 · k9mail/k-9 · GitHub
Thanks for this information 
TC contains a checkbox for choosing fle://URL (standard) or to uncheck this. Unfortunately, unchecking this setting won’t change anything. The selected file won’t be seen in K-9.
Probably it might be best to step backwards to an earlier version of K-9 
Thanks for your resonse and comments!
When I tried to test this just now, I couldn’t find the option to not use file:// URIs. Can you please describe where to find this setting?
Select a file in TC, after klicking it for two moments a list of applications will be shown to send the file two. On top of this list, top left side, there is a checkbox saying: file://URL, unchecking the box means ‘content’ instead ‘URL’. This information comes from the developer of TC.
Thanks. I don’t see this option on Android 11. But using a file:// URI doesn’t seem to be the problem on my device. I’ll investigate.
It’s working for me. The issue I had was that attachments are displayed at the bottom of the compose window. Because the keyboard is visible you have to scroll down in order to actually see the attachment box(es).
Good job 
Unfortunately I can’t confirm this on my side Sending a mail to my own is receved without attachment.
Just tested this on one of my old tablets running k-9 version 5.6 and TC version 3.2, there anything is running and responding as expected.