Also, when the page fails in this way, it would be nice if it maintained my
input data.

It turns out I accidentally selected a PDF for upload. Is there anyway to
restrict the file types shown in the file browser? If not, some validation
should be performed on file extension before submit.

I think this is pretty much the same as 96968.  If you think it's different,
please change the status.  Basically the file upload component has no way of
keeping the chosen file in the textfield.  Also I have no way to verify the
contents of a file without doing sophisticated image reading.  If you think I
should verify the file contents, please submit an enhancement request.  

Do you think file extension verification is sufficient?

*** This issue has been marked as a duplicate of 96968 ***

I think it's a bit different than 96968. File extension verification would be an
improvement. My problem was the the PDF in question looked like an image in my
file browser (I've attached it - sorry - it's another scary image).

I added file extension checking.