We are currently experiencing an issue with attachment uploads on Internet Explorer. Our developers are currently looking into this topic with a high priority.
Workaround: Attachment uploads should be working as intended on every other up-to-date browser. If you can use alternative browsers such as Chrome or Firefox in the meantime, you should be able to use this feature without any issues.
We apologize for the inconvenience. If you have any questions regarding this topic, please reach out to our support via firstname.lastname@example.org