Steps to reproduce:

1. Open any project
2. Create a "StreamingAssets" folder under Assets
3. View the folder in Finder/Explorer
4. Paste some assets into the StreamingAssets folder
5. Switch back to Unity

Unity should come up with the Importing Assets window and import the assets that were added.

Expected behaviour: Because the StreamingAssets folder has files that are going to be verbatim copies on any target machine, there's no need to import them. This is a problem if the StreamingAssets contains larger files such as videos, as it can take a while to import them.