Add the option to use FormData to encode the form elements

If the form's enctype attribute is set to "multipart/form-data",
use FormData to encode the form's elements.

lib/proto/attach-form.js

@@ -31,6 +31,22 @@ var formAction = function(el, event) {
 
   event.preventDefault()
 
+  if (el.enctype === "multipart/form-data") {
+    options.requestOptions.formData = new FormData(el)
+  }
+  else {
+    options.requestOptions.requestParams = parseFormElements(el)
+  }
+
+  el.setAttribute(attrState, "submit")
+
+  options.triggerElement = el
+  this.loadUrl(virtLinkElement.href, options)
+}
+
+function parseFormElements(el) {
+  var requestParams = []
+
   for (var elementKey in el.elements) {
     if (Number.isNaN(Number(elementKey))) {
       continue;
@@ -62,7 +78,7 @@ var formAction = function(el, event) {
         }
 
         for (var j = 0; j < values.length; j++) {
-          options.requestOptions.requestParams.push({
+          requestParams.push({
             name: encodeURIComponent(element.name),
             value: encodeURIComponent(values[j])
           })
@@ -71,10 +87,7 @@ var formAction = function(el, event) {
     }
   }
 
-  el.setAttribute(attrState, "submit")
-
-  options.triggerElement = el
-  this.loadUrl(virtLinkElement.href, options)
+  return requestParams
 }
 
 function checkIfShouldAbort(virtLinkElement, options) {