Need Advanced Answer: File Explorer.exe Thumbnails

Question

Hi,

I was unable to find any documentation that answers this question:

What/how does Explorer.exe decide which application renders a thumbnail? 

I've seen lots of replies on this forum that say "Set the default program"  or setting the default program controls what renders the image as a thumbnail but this is not true.

I have a thumbnail viewer (pretty awesome btw called ImageGlass - way better than default one that comes with Windows10) and ImageGlass is set to the default program to view all my images....

But only certain image types have a preview thumbnail, for example: 

.png has a thumb

.jpg has a thumb

.gif has no thumb

but ImageGlass is the default application that opens all 3x formats.

So what I'm looking for is, what registry keys, or what settings define which DLL, or EXE is used to render thumbnails? Does the program have to respond to certain OLE apis or respect certain flags/switches when the thumb is passed from explorer.exe to said application? 

Or can anyone point me to documentation that explains in detail, how the thumbnail rendering happens? 

I feel like there should be more stuff available than "set the default program".

Answer 1

I actually found the answer to my own question.

Posting for future reference: 

There's a shell interface called "ThumbnailProviders" 

More detail -

https://docs.microsoft.com/en-us/windows/win32/shell/thumbnail-providers

I was unable to find any documentation that answers this question:

What/how does Explorer.exe decide which application renders a thumbnail?