There is a great configuration option $wgUploadMissingFileUrl  that currently cannot be used with PageForms, because it appends the name of the file to be uploaded with wpDestFile=<filename>
So if you set
it will add &wpDestFile=
to the URL that SpecialFormEdit does not understand.
It would probably be very easy to make Special:FormEdit aware of the parameter &wpDestFile= and use this in the same way as target=
The usecase is to define a custom form for uploading files to the wiki. Works great except for red links to files. The cannot be directed to the custom form except with $wgUploadMissingFileUrl