fileUpload无法限制文件上传类型

action配置如下：我想通过allowedTypes参数限制文件上传类型只能是bmp、jpg。配置完后我上传文件的时候，任何类型都能上传。且不报错。请指教
<action name="imgupload" class="fox.fileupload.action.ImgUploadAction">
<interceptor-ref name="defaultStack"></interceptor-ref>
<interceptor-ref name="fileUpload">
<param name="allowedTypes">
image/bmp,image/jpg
</param>
<param name="maximumSize">102400</param>
</interceptor-ref>
<result name="input">index.jsp</result>
         <result name="success">upload.jsp</result>
</action>