Empty file inputs insert empty string as their submission in ajax; LMS reacts accordingly

common/lib/xmodule/xmodule/js/src/capa/display.coffee

@@ -65,11 +65,14 @@ class @Problem
     # For each file input, allow a single file submission,
     #   routed to Django 'request.FILES'
     @$('input:file').each (index, element) ->
+      if element.files[0] instanceof File
         fd.append(element.id, element.files[0])
+      else
+        fd.append(element.id, '')  # Even if no file selected, need to include input id
 
     # Simple (non-file) answers,
     #   routed to Django 'request.POST'
-    fd.append('_answers_querystring', @answers)
+    fd.append('__answers_querystring', @answers)
 
     settings = 
       type: "POST"

lms/djangoapps/courseware/module_render.py

@@ -275,13 +275,22 @@ def modx_dispatch(request, dispatch=None, id=None):
 
     post = request.POST.copy() 
 
-    # Catch the use of FormData in xmodule frontend. After this block, the 'post' dict 
-    #   is functionally equivalent before- and after- the use of FormData
+    # Catch the use of FormData in xmodule frontend for 'problem_check'. After this block,
+    #   the 'post' dict is functionally equivalent before- and after- the use of FormData
     # TODO: A more elegant solution?
-    if post.has_key('_answers_querystring'):
-        post = parse_qs(post.get('_answers_querystring'))
-        for key in post.keys():
-            post[key] = post[key][0] # parse_qs returns { key: list }
+    if post.has_key('__answers_querystring'):
+        qs = post.pop('__answers_querystring')[0]
+        qsdict = parse_qs(qs, keep_blank_values=True)
+        for key in qsdict.keys():
+            qsdict[key] = qsdict[key][0] # parse_qs returns { key: list }
+        post.update(qsdict)
+
+    # Check for submitted files
+    if request.FILES:
+        print 'Got files!'
+
+    print post.keys()
+    print request.FILES.keys()
 
     student_module_cache = StudentModuleCache(request.user, modulestore().get_item(id))
     instance, instance_module, shared_module, module_type = get_module(request.user, request, id, student_module_cache)