I did some more testing and found that the problem only happens with this nice plugin enabled: https://github.com/clonemeagain/attachment_preview
So, I wanted to report a bug at the plugin, but it has to be said that the plugin works well in the standard osTicket (I tested it). Somehow there is a conflict with osTicketAwesome. When I disable the plugin, the problem is gone.
Could you please have a look at this? The plugin is awesome too. It prevents you from downloading PDF’s, images and even audio-files (voicemails). It’s really nice and would be a shame if we have to choose between two awesome components.