[Import improved (value file-upload-field) from Golgonooza
clinton@unknownlamer.org**20081203035042
 Returns nil if a file was not uploaded or the mime-part object if one
 was
] hunk ./src/form.lisp 810
-  (client-value field))
+  (let ((client-value (client-value field)))
+    (if (not (rfc2388-binary:mime-part-p client-value))
+        nil
+        client-value)))