Merge pull request #7854 from cvium/enable_mkv_keyframe_extraction

(cherry picked from commit 2a89683e8019a196b087138531b3958d4f68789d)
Signed-off-by: crobibero <cody@robibe.ro>

MediaBrowser.Model/Configuration/EncodingOptions.cs

@@ -39,7 +39,7 @@ namespace MediaBrowser.Model.Configuration
             EnableHardwareEncoding = true;
             AllowHevcEncoding = false;
             EnableSubtitleExtraction = true;
-            AllowOnDemandMetadataBasedKeyframeExtractionForExtensions = Array.Empty<string>();
+            AllowOnDemandMetadataBasedKeyframeExtractionForExtensions = new[] { "mkv" };
             HardwareDecodingCodecs = new string[] { "h264", "vc1" };
         }