When attaching a file by drag'n drop over the composition window, special characters are replaced with spaces (I reproduced with '+', there may be other characters).

Filename : "test+test"
Error : "File ///home/jerome/test test doesn't exist"

Likewise if the special character is in the directory name.

Things work fine if adding the file using the file selection window rather than drag'n drop.

This can even be a security issue if file "test test" also exists because this is the file that is being sent.

I'm using claws-mail (3.14.1-3) from Debian Stretch repo if that matters
https://packages.debian.org/stretch/claws-mail