Make sure the files' extensions / type you're trying to upload are allowed:Admin Panel > Settings > Tickets > Attachment settingsPlease note that there are 2 attachment related settings: 1 for agents, 1 for end users.Make also sure the file size is set big enough there. The values are taken from the php.ini so when you want to increase it, you need to alter the php.ini. (And don't forget to restart/reload the webserver/php after the changes so that they got applied).Beside that, it's always good to check the error log of the webserver to get more details regarding the actual error ;)