Allow only certain downloads

Fortigate 500D v5.2.3,build670 (GA)

On our most restrictive Policy, let's call it "Common Users". I have a Web Filter Profile that blocks General Interest - Personal, Web-Based EMail. But I have a Static URL Filter that exempts our corporate web mail. The webmail works, but nobody under that profile is able to open attachments. You click the attachment and nothing ever happens. I don't see anything that stands out to me in the Log & Reports that indicates what is blocking the attachments.

I need to allow users under that policy to be able to download/view PDF and XLSX files only.