Remove parsable mime type requirement? #13

In the definition for the type attribute, the spec has:

The ASCII-encoded string in lower case representing the media type of the Blob. On getting, user agents must return the type of a Blob as an ASCII-encoded string in lower case, such that when it is converted to a byte sequence, it is a parsable MIME type [MIMESNIFF], or the empty string -- 0 bytes -- if the type cannot be determined.

There's a fair bit weird about that paragraph (e.g. 'ASCII-encoded' when the entry points which set the type already reject non-ASCII input, referring to the length of the string in bytes...). But neither Chrome nor Firefox appear to do the filtering of non-parsable MIME types:

new Blob(['yo'], {type: "te?xt/plain"}).type; // "te?xt/plain"

Even if we decide to keep the spec and file/fix implementation bugs, can we rejigger the spec so that the entry points that set type (constructors, slice()) do all of the filtering work, rather than spreading it across two parts of the spec? (Unless there's subtlety I'm missing...)

cc: @annevk @Ms2ger