Thanks for the review, Ben. Updated patches attached.

1 & 3) default_encoding -- Your two points appear to contradict each 
other slightly. What's in the updated patches is: 3.x has no encoding 
(because everything's unicode end-to-end); 2.7 attempts to apply the 
default encoding -- which is probably ascii -- to the extension and the 
mimetype and continues on error. I'm not 100% sure about this because it 
seems possible if unlikely to have a non-ascii extension / mimetype, but 
this seems like the best compromise (and is no worse than what was there 
before). Does that seem to fit the bill?

2) subkeyname[0] -- done

4) "throws EnvironmentError" -- done

5) test for .png -- done