Return warnings for file extension

[pezholio]

Needed for theodi/shared#120

[coveralls]

Coverage remained the same when pulling f4ba65c on feature-detect-file-extension into 5289b3a on master.

[JeniT]

I'm a bit suspect about this. I think it's reasonable to give a warning if the extension is .xls and the content type is text/csv or the extension is .csv and the content type is not text/csv (but I think that's picked up anyway). But I wouldn't go so far as to reject anything that has an extension other than .csv (or no extension?) -- that breaks the principle of opaque URLs.

[pezholio]

Actually, that's a good shout. I'll improve on this and open another PR.