blob() method: define behavior when get the MIME type return null

[F3n67u]

What is the issue with the Fetch Standard?

In the blob() method:

The blob() method steps are to return the result of running consume body with this and the following step given a byte sequence bytes: return a Blob whose contents are bytes and whose type attribute is the result of get the MIME type with this.

If get the MIME type return a null value when it failed to extracting a MIME type from headers:

To get the MIME type, given a Request or Response object requestOrResponse:
4. Let mimeType be the result of extracting a MIME type from headers.
5. If mimeType is failure, then return null.
6. Return mimeType.

I think In blob() method we need to either set type to an empty string or throw a TypeError like formData() does.

[annevk]

This is similar to #1693 (which links w3c/FileAPI#43). I consider this a bug in the File API for not defining the internal slot of the type getter.