added predefined mime types list and other minor ui improvements

tools/filesystem/filesystem.go

@@ -310,7 +310,7 @@ func (s *System) Serve(res http.ResponseWriter, req *http.Request, fileKey strin
 	}
 
 	// make an exception for specific content types and force a custom
-	// content type to send in the response so that it can be loaded directly
+	// content type to send in the response so that it can be loaded properly
 	extContentType := realContentType
 	if ct, found := manualExtensionContentTypes[filepath.Ext(name)]; found && extContentType != ct {
 		extContentType = ct