Glad that worked for you, I'm happy to help! Without getting too technical, the file:// prefix is the protocol of the URL. Normal websites use either http or https as their protocol. The file protocol tells Kibana that you are requesting a file that exists on the local machine, as opposed to a remote machine. A remote machine would need something like http or https to access the file.
The Kibana plugin installer can accept just a plugin name (e.g., ./bin/kibana-plugin install x-pack). When this happens, the installer will automatically try to download a plugin of that name from the Elastic servers. Since it has this capability, we need to be able to distinguish between a plugin name, and a URL. Having the protocol at the beginning helps Kibana make that distinction.