Validate file type used when editing job definition

[JasonWeill]

Problem

Found while testing #285 locally.

Create a job definition based on a notebook (.ipynb) file. Edit the job definition. Drag a Python (.py) file onto the input file drop target. Save the job definition.

The job definition gets updated, but jobs are not being created.

Proposed Solution

Allow an environment to declare which file types it can run. When editing a job definition, only accept files that the environment declares that it can run.