The Custom fields tab accommodates all information pertinent to the Case that does not have a field on the Details tab.
Information can be added to each field by clicking on the pencil icon . Here, file upload is also possible:
New custom field can be added by clicking the + New field button. When clicking in the title field to specify the type of information, a text field is displayed where the details can be typed up, as well as a field for uploading desired files.