Bug 4287 is similar to this on Windows 10, the problem I see is on an an up to date Fedora 31 using the GNOME3 desktop. If I use copy and paste then I get text like this: x-special/nautilus-clipboard copy file:///home/bdm/als.jpg in the compose window. It also sends the filename to STDOUT as far as I can tell with no STDERR output when using --debug to start Claws.
the problem code has been reverted (removed), perhaps to be re-added in future after more testing/fixing