CANVAS-114 Fix CSV content type

Might be the last chapter for https://jira.media.berkeley.edu/jira/browse/CANVAS-114

Although some browsers still upload CSV files with a MIME type of 'text/comma-separated-values', the official content type is 'text/csv', and that's what Instructure's code looks for.

I also added an explanation for not uploading a zip file, since Canvas support might ask about that sometime.