wilkinsmd:
Why does the upload addon think a .pdf file is a mime type application/download?
That depends on the browser, because the browser passes this data to a server.
So far I have tested and can confirm, that Mozilla is considering PDFs as
application/binary.
Internet Explorer, Opera, Safari for Windows, Google Chrome - as
application/pdf.
Conclusion is that obviously you're trying to test this with Mozilla, which has huge amount of bugs (as always).
However the way the files are shown in the forum is a little sloppy...
It may appear that way because of some wrong modification you did. In a default version there should be displayed
attachment_icons/pdf.gif icon near the uploaded file. You may check how it looks on
miniBB demo (I already provided some test PDF example which 100% worked on my end).