If you are unable to send attachments from your plesk panel after you have upgraded Plesk on your dedicated server or dedicated cloud VM, check the permissions of the .horde dir in /tmp. The dir permissions should be set as this:
# chown -R horde_sysuser:horde_sysgroup /tmp/.horde
# chown -R horde_sysuser:horde_sysgroup /tmp/.horde/*
# ls -al | grep horde
drwx—— 3 horde_sysuser horde_sysgroup 4096 Jun 27 11:29 .horde