Fixed an issue in UWP export caused by duplicate entry for extensions in content types file.

platform/uwp/export/export.cpp

@@ -249,7 +249,7 @@ void AppxPackager::make_content_types(const String &p_path) {
 	Map<String, String> types;
 
 	for (int i = 0; i < file_metadata.size(); i++) {
-		String ext = file_metadata[i].name.get_extension();
+		String ext = file_metadata[i].name.get_extension().to_lower();
 
 		if (types.has(ext)) {
 			continue;