Adding default MimeType guessing for uploadDir.

Hey so a friend of mine spent a couple hours banging his head against his keyboard because he didn't realize that MimeTypes weren't automatically being set. I know you have your cli which does this, but I figured since this client seems fairly popular you could add MimeType guessing as a default which is easily over-writeable...

I'm just sitting here next to him, so I haven't actually set up an AWS instance in order to test, but I think my code was decent. If you let me know, I can try to fix stuff.

Thanks!