TYPO3 Core

The thumbnail functionality can be removed as it is not needed anymore. Best description of it can be found in the CSH itself:

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The thumbnail field is used for thumbnails if you need that for your export. This is very useful when export files are used as page templates where the template selector can take advantage of the thumbnail. The format should be a web-format like jpg, gif or png and the dimensions should fit exactly the final display dimensions intended; so don't upload very large images
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