fix: clarify documentation for Blob type property to specify MIME type

[SelaseKay]

• Fix incomplete documentation for Blob 'type' property

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[srujzs]

Thanks for contributing! Editing the file directly won't work because this code is generated, including the docs. I suspect the issue is actually how we handle links: https://developer.mozilla.org/en-US/docs/Web/API/Blob/type. MIME type is a link but we omit the text in the documentation.