Fix imagefolder with one image
A dataset repository with one image and one metadata file was considered a JSON dataset instead of an ImageFolder dataset. This is because we pick the dataset type with the most compatible data file extensions present in the repository and it results in a tie in this case.

e.g. for https://huggingface.co/datasets/multimodalart/repro_1_image

I fixed this by deprioritizing metadata files in the count.

fix https://github.com/huggingface/datasets/issues/6545