Add a method to populate a default file in the UI

[manthey]

Based on some parameter in the specification, from the UI populate a default file for input.

For example, it would be nice to specify /collection/<collection name>/<folder name>/<regular expression> to get a file. If there are multiple files that match, select the most recently updated file.

Perhaps this should use the resource path endpoint, though it would be nice to allow regular expressions anywhere in the specification and to support partial path matching.

[manthey]

Added in #122.