The sending of multiple files via thunderbird has been broken for a while now. To reproduce, try to select multiple files in Nemo and select send by mail. The reason is, that the given syntax is not correct. Multiple file have to be separated by only a colon, whereas the given separator also includes a blank. See also: http://kb.mozillazine.org/Command_line_arguments_-_Thunderbird#Compose_new_mail_with_command_line Removing the blank restores the functionality.