What is the save thumbnail option appear in the save as pop up?

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What is the "save thumbnail" option appear in the bottom left of the "save
as" pop up?

Is it suppose to let the file appear as a thumbnail instead of just an icon?

If this is the case, how come it only appear in Word and Excel but not
Powerpoint?

This is because the Powerpoint files always appear as a thumbnail in my
desktop and I wish to be able to choose whether I want it to appear as an
icon or as a thumbnail.

Thanks.