#1942 - Make data_rest and file_proxy more consistent

By design, data_rest::get and file_proxy have many similarities because they both serve files. However, there are a few inconsistencies between them that could be cleaned up a little.

Most notable is the documentation. In data_rest, there's a comment that says "Note: this code is roughly duplicated in data_rest..." where it should read file_proxy, and in file_proxy there is no corresponding comment.