moved MIME type to another section; added where this metadata should be specified
moved MIME type to another section; added where this metadata should be specified

This "error" isn't a common error, so a warning about it doesn't really belong in the uber quickstart section.  But, it seems that we're collecting a few "be aware of this or that" items, so it's better if we collect these into a specific section of the instructions doc.
firebase/extensions,firebase/extensions,firebase/extensions,firebase/extensions,firebase/extensions