No thumbnail for MKV videos, only preview works

[janosorcsik]

Hi @Marginal,

I tried using the same video file from the #169 issue with the latest version of the app. Maybe the debug info will help solve the thumbnail issue.

---

QuickLook Video 3.04
macOS: Version 26.3 (Build 25D125)
Hardware: Mac14,9, Apple M2 Pro, neon=yes
Previewer: Not found
Thumbnailer: Not found
Media Formats: 3.04
Media Codecs: 3.04

file.mkv:
ContentType: 'org.matroska.mkv', subtype of 'public.audio'=no, subtype of 'public.movie'=yes
Playable: No
Thumbnail: Error Domain=QLThumbnailErrorDomain Code=0 "Could not generate a thumbnail" UserInfo={NSUnderlyingError=0x76e48d980 {Error Domain=QLThumbnailErrorDomain Code=102 "(null)"}}
Spotlight: Imported 'file.mkv' of type 'org.matroska.mkv' with plugIn /Applications/QuickLook Video.app/Contents/Library/Spotlight/QLVideo Metadata.mdimporter. 40 attributes returned

{
  "programs": [
  ],
  "stream_groups": [
  ],
  "streams": [
    {
      "codec_name": "h264",
      "profile": "High",
      "codec_type": "video",
      "codec_tag_string": "[0][0][0][0]",
      "width": 1920,
      "height": 804,
      "display_aspect_ratio": "160:67",
      "pix_fmt": "yuv420p",
      "extradata_size": 42,
      "disposition": {
        "default": 1,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    },
    {
      "codec_name": "dts",
      "profile": "DTS-HD MA",
      "codec_type": "audio",
      "codec_tag_string": "[0][0][0][0]",
      "sample_fmt": "s16p",
      "channel_layout": "5.1(side)",
      "disposition": {
        "default": 1,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    },
    {
      "codec_name": "hdmv_pgs_subtitle",
      "codec_type": "subtitle",
      "codec_tag_string": "[0][0][0][0]",
      "disposition": {
        "default": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    },
    {
      "codec_name": "hdmv_pgs_subtitle",
      "codec_type": "subtitle",
      "codec_tag_string": "[0][0][0][0]",
      "disposition": {
        "default": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    },
    {
      "codec_name": "hdmv_pgs_subtitle",
      "codec_type": "subtitle",
      "codec_tag_string": "[0][0][0][0]",
      "disposition": {
        "default": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    },
    {
      "codec_name": "hdmv_pgs_subtitle",
      "codec_type": "subtitle",
      "codec_tag_string": "[0][0][0][0]",
      "disposition": {
        "default": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      }
    }
  ],
  "chapters": [
    {
      "start_time": "0.000000",
      "end_time": "510.218000"
    },
    {
      "start_time": "510.218000",
      "end_time": "1046.545000"
    },
    {
      "start_time": "1046.545000",
      "end_time": "1532.823000"
    },
    {
      "start_time": "1532.823000",
      "end_time": "2045.085000"
    },
    {
      "start_time": "2045.085000",
      "end_time": "2693.608000"
    },
    {
      "start_time": "2693.608000",
      "end_time": "3327.908000"
    },
    {
      "start_time": "3327.908000",
      "end_time": "3743.949000"
    },
    {
      "start_time": "3743.949000",
      "end_time": "4360.314000"
    },
    {
      "start_time": "4360.314000",
      "end_time": "4850.429000"
    },
    {
      "start_time": "4850.429000",
      "end_time": "5421.833000"
    },
    {
      "start_time": "5421.833000",
      "end_time": "5769.764000"
    },
    {
      "start_time": "5769.764000",
      "end_time": "6333.285000"
    },
    {
      "start_time": "6333.285000",
      "end_time": "7564.432000"
    },
    {
      "start_time": "7564.432000",
      "end_time": "7828.320000"
    },
    {
      "start_time": "7828.320000",
      "end_time": "8152.811000"
    },
    {
      "start_time": "8152.811000",
      "end_time": "8920.128000"
    }
  ],
  "format": {
    "format_name": "matroska,webm",
    "duration": "8920.128000",
    "size": "22454628688",
    "bit_rate": "20138391",
    "probe_score": 100
  }
}

[Marginal]

Thanks again. It appears that the media extensions are in place, but they're not getting picked up for thumbnailing.
Do you have any other Quick Look thumbnail extensions that might be trying to generate thumbnails for video files?

You can list all the Quick Look thumbnail extensions with pluginkit -Avvvm | grep -C 4 com.apple.quicklook.thumbnail in Terminal. (The ones under /System likely aren't a problem).

[janosorcsik]

There are many thumbnail extensions listed by the command:

     com.apple.SceneKitQLThumbnailExtension(1.0)
	            Path = /System/Library/Frameworks/SceneKit.framework/PlugIns/SceneKitQLThumbnailExtension.appex
	            UUID = CDDF9128-5CC5-5627-AB07-DD57455A4531
	       Timestamp = 2026-02-11 20:27:20 +0000
	             SDK = com.apple.quicklook.thumbnail
	    Display Name = SceneKitQLThumbnailExtension
	      Short Name = SceneKitQLThumbnailExtension
	        Platform = macOS

--
     com.apple.RAQLThumbnailExtension(1.0)
	            Path = /System/Library/PrivateFrameworks/AssetViewer.framework/PlugIns/RAQLThumbnailExtension.appex
	            UUID = 56DC775C-E0AD-565B-A1DF-21311514A399
	       Timestamp = 2026-02-11 20:27:20 +0000
	             SDK = com.apple.quicklook.thumbnail
	    Display Name = RAQLThumbnailExtension
	      Short Name = RAQLThumbnailExtension
	        Platform = macOS

--
     com.apple.garageband10.GarageBandThumbnailExtension(1.0)
	            Path = /Applications/GarageBand.app/Contents/PlugIns/GarageBandThumbnailExtension.appex
	            UUID = F2D51118-1ACD-4E09-AD5C-AE051A88971F
	       Timestamp = 2026-02-11 20:27:43 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /Applications/GarageBand.app
	    Display Name = GarageBandThumbnailExtension
	      Short Name = GarageBandThumbnailExtension
	     Parent Name = GarageBand
--
+    com.bps.PreviewText.TextThumbnailer(1.0.9)
	            Path = /Applications/PreviewText.app/Contents/PlugIns/Text Thumbnailer.appex
	            UUID = 878708E7-CE02-4012-8416-35C162555BBE
	       Timestamp = 2026-02-11 20:27:42 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /Applications/PreviewText.app
	    Display Name = Text Thumbnailer
	      Short Name = Text Thumbnailer
	     Parent Name = PreviewText
--
     com.apple.iBooksX.BooksThumbnail(1)
	            Path = /System/Applications/Books.app/Contents/PlugIns/BooksThumbnail.appex
	            UUID = C6E9AD8C-FA3A-5BCC-96C9-950449236712
	       Timestamp = 2026-02-11 20:27:37 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /System/Applications/Books.app
	    Display Name = BooksThumbnail
	      Short Name = BooksThumbnail
	     Parent Name = Books
--
     com.apple.WatchFaceAlert.ThumbnailExtension(1.0)
	            Path = /System/Library/CoreServices/WatchFaceAlert.app/Contents/PlugIns/WatchFaceMacThumbnailExtension.appex
	            UUID = 36DC3AFC-6078-5DA5-ACFD-55FE6AE778C9
	       Timestamp = 2026-02-11 20:27:35 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /System/Library/CoreServices/WatchFaceAlert.app
	    Display Name = Watch Face
	      Short Name = WatchFaceMacThumbnailExtension
	     Parent Name = Watch Face Help
--
     com.canva.affinity.thumbnail(3.0.3)
	            Path = /Applications/Affinity.app/Contents/PlugIns/ThumbnailShareExtension.appex
	            UUID = 146FE801-1E67-41CF-B3CC-3AEABE466BC2
	       Timestamp = 2026-02-11 20:27:41 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /Applications/Affinity.app
	    Display Name = Thumbnail Extension
	      Short Name = Thumbnail Extension
	     Parent Name = Affinity
--
     com.apple.IconComposerThumbnailExtension(1.2)
	            Path = /Applications/Xcode.app/Contents/Applications/Icon Composer.app/Contents/PlugIns/Icon Composer Thumbnail.appex
	            UUID = 0194ED7E-3B7B-425F-B268-2788ED3EB520
	       Timestamp = 2026-02-27 02:15:34 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /Applications/Xcode.app/Contents/Applications/Icon Composer.app
	    Display Name = Icon Composer Thumbnail
	      Short Name = Icon Composer Thumbnail
	     Parent Name = Icon Composer
--
+    se.parotid.BookPreview.Quick-Look-Thumbnail-Extension(1.1.9)
	            Path = /Applications/Book Preview.app/Contents/PlugIns/Quick Look Thumbnail Extension.appex
	            UUID = D977CEF6-8D78-4F8B-A399-1AAAB794D278
	       Timestamp = 2026-02-11 20:27:43 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /Applications/Book Preview.app
	    Display Name = Quick Look Thumbnail Extension
	      Short Name = Quick Look Thumbnail Extension
	     Parent Name = Book Preview
--
     com.apple.OFDThumbnail(1.0)
	            Path = /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/ImageKit.framework/Versions/A/PlugIns/OFD_Thumbnail.appex
	            UUID = F0B29101-AD41-595C-94D4-4A0BFCCEB32B
	       Timestamp = 2026-02-11 20:27:31 +0000
	             SDK = com.apple.quicklook.thumbnail
	    Display Name = OFD_Thumbnail
	      Short Name = OFD_Thumbnail
	        Platform = macOS

--
     com.apple.HydraQLThumbnailExtension(1.0)
	            Path = /System/Library/PrivateFrameworks/Hydra.framework/Plugins/HydraQLThumbnailExtension.appex
	            UUID = 0FAAE9D1-F13C-574E-8A6D-F6C13F3CFA39
	       Timestamp = 2026-02-11 20:27:30 +0000
	             SDK = com.apple.quicklook.thumbnail
	    Display Name = HydraQLThumbnailExtension
	      Short Name = HydraQLThumbnailExtension
	        Platform = macOS

--
     com.apple.shortcuts.ThumbnailExtension(7.0)
	            Path = /System/Applications/Shortcuts.app/Contents/PlugIns/ThumbnailExtension.appex
	            UUID = D31E0FA5-1353-5740-8499-2898158F366E
	       Timestamp = 2026-02-11 20:27:38 +0000
	             SDK = com.apple.quicklook.thumbnail
	   Parent Bundle = /System/Applications/Shortcuts.app
	    Display Name = ThumbnailExtension
	      Short Name = ThumbnailExtension
	     Parent Name = Shortcuts
--
     com.apple.CoreHapticsTools.QLThumbnailExtension(0)
	            Path = /System/Library/PrivateFrameworks/CoreHapticsTools.framework/PlugIns/HapticsQLThumbnailExtension.appex
	            UUID = EF7CF996-5D34-5857-ADF0-BACE9F2EB6E2
	       Timestamp = 2026-02-11 20:27:23 +0000
	             SDK = com.apple.quicklook.thumbnail
	    Display Name = HapticsQLThumbnail
	      Short Name = HapticsQLThumbnail
	        Platform = macOS

[janosorcsik]

I've disabled all extensions in Settings, only QlVideo was enabled, and restarted my computer. The thumbnails are now displaying in Finder. Now, I just have to find which extension is causing the issue.

Update:
This is so strange. I've reenabled everything, restarted the computer, and the thumbnails work.

[Marginal]

I have been able to reproduce a similar issue - some parts of macOS (e.g. Finder) use the media extensions while others (e.g. QuickLook Thumbnailing process) don't. Looking through the logs it appears that the Thumbnailing process sees the media extensions, but doesn't use them to generate the thumbnail. I don't know why, but it looks like a bug in macOS.

In any case the solution appears to be to restart with QuickLook Video installed as you have found.