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) {

lib/send-request.js

@@ -51,6 +51,9 @@ module.exports = function(location, options, callback) {
         break
     }
   }
+  else if (requestOptions.formData) {
+    requestPayload = requestOptions.formData
+  }
 
   // Add a timestamp as part of the query string if cache busting is enabled
   if (options.cacheBust) {

tests/lib/proto/attach-form.js

@@ -152,3 +152,33 @@ tape("test form elements parsed correctly", function(t) {
 
   t.end()
 })
+
+tape("test form.enctype=\"multipart/form-data\"", function(t) {
+  t.plan(4)
+
+  var form = document.createElement("form")
+  form.enctype = "multipart/form-data"
+  var input = document.createElement("input")
+  input.name = "input"
+  input.value = "value"
+  form.appendChild(input)
+
+  var pjax = {
+    options: {},
+    loadUrl: function(href, options) {
+      t.equals(options.requestOptions.requestParams.length, 0, "form elements not parsed manually")
+      t.true(options.requestOptions.formData instanceof FormData, "requestOptions.formData is a FormData")
+      t.equals(Array.from(options.requestOptions.formData.entries()).length, 1, "correct number of FormData elements")
+      t.equals(options.requestOptions.formData.get("input"), "value", "FormData element value set correctly")
+    }
+  }
+
+  attachForm.call(pjax, form)
+
+  form.action = window.location.protocol + "//" + window.location.host + "/internal"
+
+  trigger(form, "submit")
+  // see loadUrl defined above
+
+  t.end()
+})