Documented FileType variables

Author: wouterj

reference/forms/types/file.rst

@@ -100,3 +100,12 @@ These options inherit from the :doc:`form </reference/forms/types/form>` type:
 .. include:: /reference/forms/types/options/error_mapping.rst.inc
 
 .. include:: /reference/forms/types/options/mapped.rst.inc
+
+Form Variables
+--------------
+
+======== ========== ==========================================================================
+Variable Type       Usage                                       
+======== ========== ==========================================================================
+type     ``string`` The type variable is set to ``file``, in order to render like a file input
+======== ========== ==========================================================================