[w3c/FileAPI] What should happen when you load a blob URL in an iframe? (#82)

Subsection [Response Headers](https://w3c.github.io/FileAPI/#processing-media-types) of section [Dereferencing Model for Blob URLs](https://w3c.github.io/FileAPI/#requestResponseModel) of File API does not describe what to do when dereferencing a Blob URL that was constructed from raw data and does not have a type. What should we do?

**Example**

Open a page with the following markup:

```
<!DOCTYPE html>
<html>
<body>
<script>
var frame = document.createElement("iframe");
frame.src = URL.createObjectURL(new Blob(["<h1>Hello</h1>"]));
document.body.appendChild(frame)
</script>
</body>
</html>
```

What should happen?

Here is what various browsers do:

Browser        |  Observed behavior
------------ | -------------
Chrome Canary for Mac 61.0.3115.0 (Official Build) canary (64-bit) | Shows an iframe with `<h1>Hello</h1>` (verbatim) (i.e. renders content as plain text)
Firefox for Mac 53.0.3 (64-bit) | Shows an iframe with <h1>Hello</h1> (rendered as HTML)
Safari 10.1 (12603.1.24) | Downloads the contents associated with the blob URL to a file named "unknown"
Edge 38.14393.1066.0 (EdgeHTML 14.14393) | Shows an empty iframe
Internet Explorer 11 (11.1198.14393.0) | Shows an empty iframe

**Additional remarks:**

For comparison, modify the above example to construct the Blob with type "text/html" and open this page:

```
<!DOCTYPE html>
<html>
<body>
<script>
var frame = document.createElement("iframe");
frame.src = URL.createObjectURL(new Blob(["<h1>Hello</h1>"], {type: "text/html"}));
document.body.appendChild(frame)
</script>
</body>
</html>
```

Here is what the above browsers do:

Browser        |  Observed behavior
------------ | -------------
Chrome Canary for Mac 61.0.3115.0 (Official Build) canary (64-bit) | Shows an iframe with <h1>Hello</h1> (rendered as HTML)
Firefox for Mac 53.0.3 (64-bit) | Shows an iframe with <h1>Hello</h1> (rendered as HTML)
Safari 10.1 (12603.1.24) | Shows an iframe with <h1>Hello</h1> (rendered as HTML)
Edge 38.14393.1066.0 (EdgeHTML 14.14393) | Shows an empty iframe
Internet Explorer 11 (11.1198.14393.0) | Shows an empty iframe

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Received on Friday, 2 June 2017 17:52:55 UTC