I have to add that if I select "in-line" instead of "as attachment" only the file name appears at the destination.

The mentioned version 1248.4.1esr (64-bit) corrupted all the previous mail files and I do not recommend its installation or use.

I believe this corruption issue was caused by having the two apps working side-by-side. Probably mail files are not being locked during writing, which allows simultaneous writing, which in turn corrupted the files.

Latest update:

After unistalling the flatpak version, which was causing the problem, I had the mozilla-team thunderbird-next repo installed:

sudo add-apt-repository ppa:mozillateam/thunderbird-next sudo apt update

And created the file:

/etc/apt/preferences.d/mozilla

with the contents:

Package: * Pin: release o=LP-PPA-mozillateam Pin-Priority: 1001

Package: firefox Pin: version 1:1snap* Pin-Priority: -1

Package: thunderbird Pin: version 1:1snap* Pin-Priority: -1

Installed thunderbird 128

128.4.2esr (64-bit) was installed and the attachment problem gone. This version only allows attachments, not in-line. Do not install it from flatpak

Now everyhing apparently runs as expected.