form: don't pass over additional file checks if there are no file (#6239)

wcs/qommon/form.py

@@ -598,6 +598,10 @@ class FileWithPreviewWidget(CompositeWidget):
             temp = session.tempfiles[token]
             self.value = session.get_tempfile_content(token)
 
+        if self.value is None:
+            # there's no file, the other checks are irrelevant.
+            return
+
         if self.max_file_size:
             # validate file size
             file_size = os.path.getsize(self.value.fp.name)