Add content-type to response body info

fetch.bs

@@ -346,6 +346,9 @@ following <a for=struct>items</a>: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]]
  <dt><dfn export for="response body info" id="fetch-timing-info-decoded-body-size">decoded
  size</dfn> (default 0)
  <dd>A number.
+<dt><dfn export for="response body info" id="response-body-info-content-type">content
+ type</dfn> (default empty string)
+ <dd>A string.
 </dl>
 
 <p>To
@@ -4518,6 +4521,14 @@ steps:
      <var>response</var>'s <a for=response>has-cross-origin-redirects</a> is true; otherwise
      <var>response</var>'s <a for=response>status</a>.
 
+     <li><p>Let <a for=response>body info</a>'s <a for="response body info">content type</a> be the
+     empty string if <var>response</var>'s <a for=response>type</a> is "<code>opaque</code>" or
+     if <var>fetchParams</var>'s <a for="fetch params">request</a>'s <a for=request>mode</a> is
+     "<code>navigate</code>" and <var>response</var>'s
+     <a for=response>has-cross-origin-redirects</a> is true; otherwise result of
+     <a for="header list">extracting a MIME type</a> from <var>response</var>'s
+     <a for=response>header list</a>.
+
      <li><p>If <var>fetchParams</var>'s <a for="fetch params">request</a>'s
      <a for=request>initiator type</a> is not null, then
      <a for=/>mark resource timing</a> given <var>timingInfo</var>, <var>request</var>'s