Feature suggestion:

When you want to use page forms for uploading files in your wiki (which btw. works great, you specify a form that enters pages in the File: namespace), you could redirect the "Upload file" link with $wgUploadNavigationUrl to your form, e. g. Form:Fileupload

However, if you want to use $wgUploadMissingFileUrl to point red links to files to the form, you could set $wgUploadMissingFileUrl = '/Special:Edit form/Fileupload/'; but the problem is, that the form does not recognise the (?|&)wpDestFile=<filename> paramater that will be added to the URL according to Manual:$wgUploadMissingFileUrl. So the resulting link is

Special:FormEdit/Fileupload/?wpDestFile=Filename.jpg

when it should be

Special:FormEdit/Fileupload/Filename.jpg

for page forms.

So this is no bug, but a feature request. It should be very easy to make Page Forms aware of the (?|&)wpDestFile=<filename> mechnism in $wgUploadMissingFileUrl and would be a very powerful feature!