https://www.icyphoenix.com/viewtopic.php?f=26&t=7021&p=48278#p48278
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Mighty Gorgon
Sun 14 Mar, 2010 09:51

Re: Only Png Mid Thumbnails Cached, No Jpg Thumbnails
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Are you sure your JPG files are larger than the mid thumb size specified in ACP?

If actual JPG file is smaller than thumb size it should not be cached.