File extension unknown by the browser fails to trigger server side mimetype detection upon drag and drop

Steps to reproduce:

1- register a mimetype definition for ".myextension"
2- map this mimetype to a specific FileManager plugin
3- drop a file ending ".myextension" in a workspace

Expected results: the plugin is called
Observed results: the default FileManager file importer is used instead.

The reason is that chrome is passing the empty string "" as mimetype which is not mapped to anything. The FileManagerService should be more robust and trigger a server side mimetype detection in that case.