Add "worksheet file" pattern for editing batches

lets user download a worksheet, edit, then upload back to update the batch

tailbone/forms/core.py

@@ -623,6 +623,9 @@ class Form(object):
             if 'required' in kwargs and not kwargs['required']:
                 kw['missing'] = colander.null
             self.set_node(key, colander.SchemaNode(deform.FileData(), **kw))
+            # must explicitly replace node, if we already have a schema
+            if self.schema:
+                self.schema[key] = self.nodes[key]
         else:
             raise ValueError("unknown type for '{}' field: {}".format(key, type_))