Don't raise KeyError if a non-required file wasn't uploaded.

examples/megacorp_acc/job_import_employees.sh

@@ -1,5 +1,10 @@
 #!/bin/sh
 
+if [ -z "$csv_file" ]; then
+    echo "No file specified" >&2
+    exit 1
+fi
+
 {
     echo ".separator ,"
     echo ".import $csv_file employee"

src/formdefinition.py

@@ -225,7 +225,10 @@ class FormDefinition(object):
         """
         Validate a form field of type 'file'.
         """
+        try:
             value = form_values[field_def['name']]
+        except KeyError:
+            return None
         field_name = field_def['name']
         upload_fname = form_values[u'{0}__name'.format(field_name)]
         upload_fname_ext = os.path.splitext(upload_fname)[-1].lstrip('.')