Clarify package data algorithm for FormData

Fixes #392.

fetch.bs

@@ -23,7 +23,9 @@ Translate IDs: typedefdef-bodyinit bodyinit,typedefdef-responsebodyinit response
 <script src=https://resources.whatwg.org/dfn.js defer id=head></script>
 
 <pre class=anchors>
-url:https://xhr.spec.whatwg.org/#concept-formdata-entry;spec:XHR;type:dfn;text:entries
+url:https://xhr.spec.whatwg.org/#concept-formdata-entry;spec:xhr;type:dfn;
+    text:entry
+    text:entries
 url:https://tools.ietf.org/html/rfc7230#section-3.1.1;text:method;type:dfn;spec:http
 url: https://tools.ietf.org/html/rfc7230#section-3.2;text:field-name;type:dfn;spec:http
 url: https://tools.ietf.org/html/rfc7230#section-3.2;text:field-content; type:dfn;spec:http
@@ -4360,16 +4362,34 @@ runs the associated steps:
   run these substeps:
 
   <ol>
-   <li><p>Parse <var>bytes</var>, using the value of the `<code>boundary</code>` parameter from
-   <var>MIME type</var> and <a>utf-8</a> as encoding, per the rules set forth in
-   <cite>Returning Values from Forms: multipart/form-data</cite>.
-   [[!RFC7578]]
+   <li>
+    <p>Parse <var>bytes</var>, using the value of the `<code>boundary</code>` parameter from
+    <var>MIME type</var>, per the rules set forth in
+    <cite>Returning Values from Forms: multipart/form-data</cite>. [[!RFC7578]]</p>
+
+    <p>Each part whose `<code>Content-Disposition</code>` header contains a `<code>filename</code>`
+    parameter must be parsed into an <a>entry</a> whose value is a {{File}} whose content is the
+    content of the part. The {{File/name}} attribute of the {{File}} must have the value of the
+    `<code>filename</code>` parameter of the part. The {{Blob/type}} attribute of the {{File}} must
+    have the value of the `<code>Content-Type</code>` header of the part if the part has such
+    header, and `<code>text/plain</code>` (the default defined by [[!RFC7578]] section 4.4)
+    otherwise.</p>
+
+    <p>Each part whose `<code>Content-Disposition</code>` header does not contain a
+    `<code>filename</code>` parameter must be parsed into an <a>entry</a> whose value is the
+    <a>UTF-8 decoded</a> content of the part. <span class=note>This is done regardless of the
+    presence or the value of a `<code>Content-Type</code>` header and regardless of the presence or
+    the value of a `<code>charset</code>` parameter.</span></p>
+
+    <p class=note>A part whose `<code>Content-Disposition</code>` header contains a
+    `<code>name</code>` parameter whose value is `<code>_charset_</code>` is parsed like any other
+    part. It does not change the encoding.</p>
+   </li>
 
    <li><p>If that fails for some reason, then <a>throw</a> a <code>TypeError</code>.
 
-   <li><p>Return a new {{FormData}} object, appending each entry,
-   resulting from the parsing operation, to
-   <a>entries</a>.
+   <li><p>Return a new {{FormData}} object, appending each <a>entry</a>, resulting from the parsing
+   operation, to <a>entries</a>.
   </ol>
 
   <p class=XXX>The above is a rough approximation of what is needed for