Merge feature SOL-20

This is a pull request to merge the SOL-20 feature (https://openedx.atlassian.net/browse/SOL-20).
Yes @andy-armstrong - here it is not being used to perform filtering, here we are showing the file extension in the table - perhaps we should be changing the name of this function to get_extension, or even pass the extension down after parsing it using os.path.splitext on the server side (@dcikatic)? @marcotuts - does it seem odd that we're filtering by file classification, and showing file extension in the column? Seems ok to me, but seems relevant to bring it up here.