Provide examples of Custom Formatters

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Some sort of image preview of the results of a Custom Formatter in use is essential, otherwise there is no way of knowing what it really does unless the description is extremely clear or the users are able to understand the code.

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Initial work for this has been done locally with two screenshot fields provided before/after or input/output.

Custom validation needs to be added so that if one image is uploaded the other must also be uploaded.