I cannot use plugins (in my case replaygain) if the file has not been looked up or not found.
However I expected to use replaygain also for files in the unclustered/clustered section.

Steps: 
1. Click on "add files" and add any song, that has not been looked up with metabrainz.
2. Right-Click and look for the "plugins" submenu
3. It's not there

Expected:

Replaygain can be used on any files.