Specify file as concat when extracting attachments

MediaBrowser.MediaEncoding/Attachments/AttachmentExtractor.cs

@@ -139,7 +139,8 @@ namespace MediaBrowser.MediaEncoding.Attachments
 
             var processArgs = string.Format(
                 CultureInfo.InvariantCulture,
-                "-dump_attachment:t \"\" -y -i {0} -t 0 -f null null",
+                "-dump_attachment:t \"\" -y {0} -i {1} -t 0 -f null null",
+                inputPath.EndsWith(".concat\"", StringComparison.OrdinalIgnoreCase) ? "-f concat -safe 0" : string.Empty,
                 inputPath);
 
             int exitCode;