Fix false positive detection

PNGs or GIFs can currently be improperly detected as JPEGs, this fixes that:
1st it checks for mime on the blob
2nd it checks for data:image/jpeg for dataURLs
3rd it does a check for .jpg or .jpeg for provided string srcs

Otherwise, any transparent PNGs or GIFs will also be detected as "squashed".

Thanks for the great library!