Allow PDFs KB Articles to be Revised

Currently, KB articles can be made by directly uploading a PDF.

However, if the PDF changes (say, it is a vendor manual and a new revision of the manual is released) the PDF KB cannot have the source PDF replaced (neither by Web GUI nor by API). The only option is to archive or delete the existing KB and create a new PDF KB, and recreate all relations, comments, and other metadata associated with the existing KB.

In the desired state, a PDF KB article could have the source PDF changed by web GUI or by API so that the KB can be kept up to date while preserving all relationships to the KB.