The "File" field is a pathname of a file (or directory) containing information related to the request. When empty, it isn't displayed. To set it, it must first be selected from the "Can be set" section of the Update Request form.
The pathname can be any valid path.
Access control on the files should be commensurate with that on the corresponding ST item. I.e. typically no general read permission.
In practice, this is an unused field, as most use attachments to the email gateway as a way of associating files with service records, and that (unfortunately) places the files elsewhere.
When specifying this field in a customized sort or display, use the name "file".