Drag from file browser to dock panel no longer works

[ellisonbg]

In previous versions of JupyterLab, you could drag a file from the file browser to the dock panel and pick where the file would be opened. This no longer works in master. That might have been a deliberate decision, but want to make sure before we release 3.0

[ianhi]

This is may be related to #8205

Also what you describe is also broken for me on version 2.1.1

[blink1073]

This was an oversight when we changed from phosphor -> lumino. This line

jupyterlab/packages/filebrowser/src/listing.ts

Line 198 in be73243

const FACTORY_MIME = 'application/vnd.phosphor.widget-factory';

needs to updated to accommodate the expected mimetype