Add download link to file uploads instead of [FILE] in exports

[m-martinez]

Currently an export data file will render a BLOB type as [FILE] in the column. We did this to avoid attaching binary data to a CSV file, as it would just look like gibberish.

It would be nice if instead we render a download link so that the user can view/download the contents, (e.g. http://somestudy.org/reports/file/12345)