add multipart/form-data support to http request node

nodes/core/io/21-httprequest.html

@@ -129,7 +129,18 @@
     <p>If <code>msg.payload</code> is an Object, the node will automatically set the content type
     of the request to <code>application/json</code> and encode the body as such.</p>
     <p>To encode the request as form data, <code>msg.headers["content-type"]</code> should be set to <code>application/x-www-form-urlencoded</code>.</p>
-
+    <p>For form-based file uploads, some services require that <code>msg.headers["content-type"]</code> be set to <code>multipart/form-data</code>.
+    To construct <code>msg.payload</code> for this type of request, use the following structure and substitue the <code>KEY</code>, <code>DATA</code>, and <code>FILENAME</code> 
+    values for your own.
+<pre><code>req.formData = { 
+    KEY: { 
+        value: DATA, 
+        options: { 
+            filename: ‘FILENAME’ 
+        } 
+    } 
+}
+</code></pre>
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