Sorry, but I don't think that this is an osTicket issue when the employee can only upload photos and not any documents. Despite I don't own an iPhone not tried to upload a document with my (Windows) phone, I think it's an issue of the iPhone that it opens automatically the pictures/albums to select one for upload and not any kind of file.
Please correct me if I am wrong and add please not info about your osTicket or try it with a different phone and add if it behaves in the same manner or different.