Provide basic ability for data providers to upload new file(s) to datasets.

Initial scope does not include ability to "delete" or "replace" file.

Test/Acceptance:

Test for informative error when file exists and/or there is a file upload problem.
Test multiple file uploads. Single failure should fail entire set.
Test a "Readme.txt" file upload.
Be able to find uploaded file(s) on file system without much effort.