struts2 文件上传时，某次上传失败之后的所有上传都返回input，不跳到action

在做文件上传时，不加限制上传任意类型文件都成功，但是如果加了“allowedTypes”限制文件类型后，上传正确类型成功，上传错误类型失败，这些都是理所当然的结果。问题是上传错误类型失败后，再上传正确的文件，也跳转到input视图，不经过action。如限制只能上传gif格式文件，第一次上传gif图片返回success；第二次上传非gif文件，返回input；第三次上传gif文件，还是返回input，怎么解决？
代码如下：
jsp页面：    <s:form action ="fileUpload" method ="POST" enctype ="multipart/form-data" >
             <s:file name ="myFile" label ="Image File" />
             <s:textfield name ="caption" label ="Caption" />       
             <s:submit/>
action：
package ss2h.Action;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Date;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.ActionSupport;

public class FileUploadAction extends ActionSupport {
    private static final long serialVersionUID = 572146812454l ;
    private static final int BUFFER_SIZE = 16 * 1024 ;
  
    private File myFile;
    private String contentType;
    private String fileName;
    private String imageFileName;
    private String caption;
  
    public void setMyFileContentType(String contentType) {
        this .contentType = contentType;
   }
  
    public void setMyFileFileName(String fileName) {
        this .fileName = fileName;
   }
      
    public void setMyFile(File myFile) {
        this .myFile = myFile;
   }
  
    public String getImageFileName() {
        return imageFileName;
   }
  
    public String getCaption() {
        return caption;
   }

    public void setCaption(String caption) {
        this .caption = caption;
   }
  
    private static void copy(File src, File dst) {
        try {
           InputStream in = null ;
           OutputStream out = null ;
            try { 
           
            if (dst.getParentFile() != null
&& dst.getParentFile().exists() == false) {
if (dst.getParentFile().mkdirs() == false) {
throw new IOException("Destination '" + dst
+ "' directory cannot be created");
}
}
           
            //文件不会自动保存到服务器，必须通过流操作
               in = new BufferedInputStream( new FileInputStream(src), BUFFER_SIZE);
               out = new BufferedOutputStream( new FileOutputStream(dst), BUFFER_SIZE);
                byte [] buffer = new byte [BUFFER_SIZE];
                while (in.read(buffer) > 0 ) {
                   out.write(buffer);
               }
           } finally {
                if ( null != in) {
                   in.close();
               }
                if ( null != out) {
                   out.close();
               }
           }
       } catch (Exception e) {
           e.printStackTrace();
       }
   }
  
    private static String getExtention(String fileName) {
        int pos = fileName.lastIndexOf(".");
        return fileName.substring(pos);
   }

   @Override
    public String execute()     {       
       imageFileName = new Date().getTime() + getExtention(fileName);
       File imageFile = new File(ServletActionContext.getServletContext().getRealPath( "/UploadImages" ) + "/" + imageFileName);
       copy(myFile, imageFile);
       System.out.println("haha");
        return SUCCESS;
   }
  
}

struts.xml：
<package name ="fileUploadDemo" extends ="struts-default" >
        <action name ="fileUpload" class ="FileUploadAction" >           
              <!-- 验证上传文件的类型 -->    
            <interceptor-ref name ="defaultStack">     
                <param name ="fileUpload.allowedTypes">image/gif</param>      
            </interceptor-ref>                     
            <result name ="input"> /fail.jsp </result >
            <result name ="success"> /success.jsp </result >
        </action >
    </package >
applicationContext.xml：
<bean id="FileUploadAction" class="ss2h.Action.FileUploadAction">
</bean>

web.xml：
<filter>
  <filter-name>struts2</filter-name>
  <filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class>
  </filter>
  <filter-mapping>
  <filter-name>struts2</filter-name>
  <url-pattern>/*</url-pattern>
  </filter-mapping>
 
  <filter >
        <filter-name > struts-cleanup </filter-name >
        <filter-class >
            org.apache.struts2.dispatcher.ActionContextCleanUp
        </filter-class >
    </filter >  
    <filter-mapping >
        <filter-name > struts-cleanup </filter-name >
        <url-pattern > /* </url-pattern >
    </filter-mapping >
   
  <listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>