I wrote a thumbnail extension handler for jpeg files for windows 10. I registered the handler to handle ".jpg" files. Thumbnails for ".jpg" files are displayed correctly. However, I noticed that on some ".jpg" files my handler is being called and on others it doesn't.
I understand now that the reason my handler is not being called is because the non-working jpeg files contain an embedded thumbnail inside them. I tried to remove the embedded thumbnail using exiftool:
exiftool.exe -thumbnailimage= c:\test\test.jpg
exiftool.exe -photoshopthumbnail= c:\test\test.jpg
Once the embedded thumbnail is removed, my handler is being successfully called by explorer. If explorer is not using the thumbnail handler to extract embedded thumbnails, how does it do it? Which handler retrieves it?