Recognise file type of uploads with upper case file names

The content type used when sending uploads to the user is based on
the file extension.  However, previously it didn't correctly
recognise uppercase file extensions, causing problems like this[1].

[1] https://www.pivotaltracker.com/story/show/44879513
Merge pull request #265 from alphagov/uppercase-file-extensions-not-recognised

Recognise file type of uploads with upper case file names