Change MIME type for PNG images to image/png

This is the official MIME type assigned by the IANA (see http://www.iana.org/assignments/media-types/image/png).

src/webui/httpresponsegenerator.cpp

@@ -60,7 +60,7 @@ void HttpResponseGenerator::setContentTypeByExt(const QString& ext) {
 		return;
 	}
   if (ext == "png") {
-		setContentType("image/x-png");
+		setContentType("image/png");
 		return;
 	}
 }