Allow file overwrite

[nhammond]

Reproduction steps: Export a run (or any file with metadata), flush the database, re-import. Because it preserves the relative file paths, and the server defaults to the same file root, and the original file was not deleted, the import raises an error saying file already exists. Refusing to overwrite is useful, but it would be nice if I could override this as a user and force the old file to be over-written.