PDF Preview in Tor Browser isn't always available

Probably, the reason is legacy/trac#17207 (moved) (Testing navigator.mimeTypes for known names can reveal info and increase fingerprinting risk).

Example: https://download.microsoft.com/download/9/9/4/994592CB-C248-464F-93A6-A50E339BE19B/Windows%208%20Security%20-%20ASLR.pdf