SymptomsHorde webmail from Parallels Plesk Panel distribution sends a message with an attachment with a non-ASCII name. The resulting message may contain a broken attachment with a different name, depending on the mail client used to view it.
CauseHorde webmail from Parallels Plesk Panel distribution does not properly handle appropriate filename conversion for non-ASCII charsets.
ResolutionThe problem has been fixed a long time ago, update your Parallels Plesk Panel installation to install the fix.
WorkaroundIf updating did not help in your case, put the Part.php file of the appropriate version (inside the corresponding directory) from the attached archive into one of the following locations:
Parallels Plesk Panel for Linux -- /usr/share/psa-horde/lib/Horde/MIME/Part.php
Parallels Plesk Panel for Windows -- %plesk_vhosts%\webmail\horde\lib\Horde\MIME\Part.php