Посмотрите на Form.onFileUploadException
:
if (e instanceof SizeLimitExceededException)
{
// Resource key should be <form-id>.uploadTooLarge to
// override default message
final String defaultValue = "Upload must be less than " + getMaxSize();
String msg = getString(getId() + '.' + UPLOAD_TOO_LARGE_RESOURCE_KEY, Model.ofMap(model), defaultValue);
error(msg);
}