[S2]can I skip file upload validation when I did not upload a file?

[钟原froast]

<interceptor-ref name="defaultStack">
                  <param name="fileUpload.allowedTypes
">image/png,image/gif,image/jpeg</param>
                 <param name="fileUpload.maximumSize">102400</param>
                 <param name="validation.excludeMethods">*</param>
                 <param name="validation.includeMethods
">saveCertificate</param>
</interceptor-ref>

the upload file is not nessary, so i want it to be validated when I upload a
file, and skip validation when I did not upload a file, how should I do?