For any following along, here's what I shared with Bill a few weeks back:
You could dig inside the .epr and look at:
In this case, 1299148630 == 0x4D6F6F56 == ‘MooV’ == .mov
It gets a little more tricky when you have multiple possibilities for file extensions.
For exporters, there’s the selector exSelQueryExportFileExtension, which is called by the host app. However, the info returned by the exporter here isn’t currently passed along to export controllers, which is somewhat of a shortcoming. So there’s currently no good way through the API. This would probably belong as a feature request, if you want to see this done in the future.