Update comment to be accurate.

submit.go

@@ -435,7 +435,10 @@ func saveLogPart(logNum int, filename string, reader io.Reader, reportDir string
 	// some clients use sensible names (foo.N.log), which we preserve. For
 	// others, we just make up a filename.
 	//
-	// Either way, we need to append .gz, because we're compressing it.
+	// We append a ".gz" extension if not already present, as the final file we store on
+	// disk will be gzipped. The original filename may or may not contain a '.gz' depending
+	// on the client that uploaded it, and if it was uploaded already compressed.
+
 	var leafName string
 	if logRegexp.MatchString(filename) {
 		leafName = filename