* fix image serializing
* If get_format_mimetype does not work, look up format in Image.MIME
Throw a warning if the image format does not have an associated MIME type and
ultimately fallback to image/png and let the browser figure it out.
* test_media: end to end serialization of PIL images
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Co-authored-by: Masen Furer <m_github@0x26.net>
