Deduce mimeType for non-text files obtained through USDA connector

There are some tiff files coming through this connector which are mistakenly labelled as mimeType: image/jpg (e.g. http://eol.org/api/data_objects/1.0/1386367.json). This code change may correct that.

I am unsure why the current code has lines 363-371 commented out, though.