Files: accept more filetypes and increase security

Request is to allow more types of document formats to be uploaded into STR.

There would be some increased storage costs due to the rastered image affect, but as many of the documents are scanned images to begin with, it should not be significant.

A service like https://dangerzone.rocks/ would be a secure way of creating PDFs that would not harm examiner's systems, providing a safe and secure approach to accepting more document types, without incurring downstream compromises or dependencies.