It seems a number of people have had this problem. 

This thread suggests that he problem is being caused by Sketch. Is this a program you have? 
"Had the same issue here (macOS Mojave 10.14). Updating Sketch and rebooting solved the issue for me."


On 9/13/19, 11:46 AM, "SciArt-L Discussion List-for Natural Science Illustration- on behalf of Lynette Cook" <[log in to unmask] on behalf of [log in to unmask]> wrote:

    Since upgrading to Mohave I have noticed a problem with image file thumbnails: most of the time I see a generic thumbnail for the file type (JPEG, PSD) - not the image itself. So I can’t remember what a lot of these images are unless I open the file! Anybody know what’s causing this? A Google search indicates that some people fixed the issue by deleting Sketch or a Google app of some kind, yet I have neither of these.

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