Well, browsers differ on what they'll show as far as filetype. I was completely surprised when doing some user testing on a site where we had links to PDFs open "forward" (no target-blank). Since we did the testing on the browsers and OSes the users were using, they kept accidentally closing their browsers... because the PDF was opened in their browser, and they had expected a new window because that's the behaviour they always see/get at all other sites they visit. This was why I ended up changing PDF links to new tabs/windows.
The reasons I and the other devs had not noticed this was because our browsers and OSes downloaded PDFs like files, leaving the browser pages unchanged.
If someone knows how that all works, please post! I'd be interested in knowing (also partially because all teh Google stuff seems horribly inaccessible to screen readers unless you're using ChromeVox (gee, wonder why...)).