jspSmart下载文件出错

用jspSmart写了程序如下：
<%@ page contentType="text/html;charset=gb2312" import="com.jspsmart.upload.*" %><%
    SmartUpload su = new SmartUpload();
su.initialize(pageContext);
su.setContentDisposition(null);
su.downloadFile("/upload/Readme.txt");%>
出现了奇怪的问题：
可以下载Readme.txt文件，但是当我在upload文件夹下新建其他文件后，却不能下载，例如我将最后一行改为：
su.downloadFile("/upload/Readme.doc");%>（该文件的确位于正确文件夹下）
就报错：
严重: Servlet.service() for servlet jsp threw exception
org.apache.jasper.JasperException: An exception occurred processing JSP page /task/download.jsp at line 52:     SmartUpload su = new SmartUpload();
3:  su.initialize(pageContext);
4:  su.setContentDisposition(null);
5:  su.downloadFile("/upload/Readme.doc");%>
Stacktrace:
at org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:524)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:423)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:263)
at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:852)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:584)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1508)
at java.lang.Thread.run(Unknown Source)
请问这是怎么一回事啊？