This user has the permission Attachments (Add/Delete), but is impossible include files.
The box you are showing there is asking you to select an existing file already attached to the page. There are no attachments yet, so it's empty.
Try attaching the file via the attachment link in the menu.
User admin in Attachments:
User no admin in Attachments:
Note: This problem arose after the Confluence update.
Ok, that makes sense. Go to Space Admin and check the permissions. Your second user does not have "attachment" rights from what I can see.
It looks like you're new here. Sign in or register to get started.