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I don't know what that means!

Sorry for being brief. An extension is the part of the file name after the . (dot). As an example if a thumbnail is stored on your computer as -- thumbnail.jpg. The jpg is the extension. There are several extensions used for thumbnails such as JPEG, BMP, TIFF, PDF, AVI etc. The problem might be that the platform you are trying to upload the thumbnail to may not recognize the extension of the thumbnail you are trying to upload. Does this make sense?

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