macOS How to define the WebPluginMIMETypesFilename in the plist of a safari webplugin proje

I want to know the defination of WebPluginMIMETypesFilename, if my project is SafariPlugins. target extension is webplugin, My WebPluginMIMETypesFilename is com.SafariPlugIns.webplugin.plist? and what is the webPluginMIMETYpes, I just know when safari lunch , it will test the webplugin if have the webPluginMIMETYpes or not. and then load the plugin , but it is what to do ? and it's definition?

and another question is : if I define the the Principal class of plist my plugin class name:SafariPlugIn, and if the App will begin in that class firstly. and in the plugin class the method:

* (NSView *)plugInViewWithArgumentsNSDictionary *)arguments

will begin firstly?