MDL-60001 core_files: Use local file for gzopen

The gzopen method does not appear to support
file streams.

lib/filestorage/file_system.php

@@ -478,7 +478,12 @@ abstract class file_system {
      * @return resource file handle
      */
     public function get_content_file_handle(stored_file $file, $type = stored_file::FILE_HANDLE_FOPEN) {
-        $path = $this->get_remote_path_from_storedfile($file);
+        if ($type === stored_file::FILE_HANDLE_GZOPEN) {
+            // Local file required for gzopen.
+            $path = $this->get_local_path_from_storedfile($file, true);
+        } else {
+            $path = $this->get_remote_path_from_storedfile($file);
+        }
 
         return self::get_file_handle_for_path($path, $type);
     }

lib/filestorage/tests/file_system_test.php

@@ -904,8 +904,8 @@ class core_files_file_system_testcase extends advanced_testcase {
         $filecontent = 'example content';
         $file = $this->get_stored_file($filecontent);
 
-        $fs = $this->get_testable_mock(['get_remote_path_from_storedfile']);
-        $fs->method('get_remote_path_from_storedfile')
+        $fs = $this->get_testable_mock(['get_local_path_from_storedfile']);
+        $fs->method('get_local_path_from_storedfile')
             ->willReturn(__DIR__ . "/fixtures/test.tgz");
 
         // Note: We are unable to determine the mode in which the $fh was opened.