Allow video uploads

Summary: Allows video uploads from native iOS by using the proper network handler, `RCTAssetsLibrary` rather than `RCTImageLoader`. Currently, uploading a camera roll video via its `assets-library://` URI would upload the first image frame. This fixes it by checking if the `assets-library://` URI is of the extension `MOV`, and if so, declines to process it from `RCTImageLoader`.

Reviewed By: javache

Differential Revision: D5223351

fbshipit-source-id: 8de92c5bb72acb02ed4d9fb92d69f68c8b5d1b36