Content-Disposition 属性是作为对下载文件的一个标识字段，在rfc2616
http://www.rfc-editor.org/rfc/rfc2616.pdf
章节19.5 Additional Features中
有介绍，具体介绍请看
http://www.rfc-editor.org/rfc/rfc1806.txt
字段介绍如下：
disposition := "Content-Disposition" ":"
disposition-type
*(";" disposition-parm)
disposition-type := "inline"
/ "attachment"
/ extension-token
; values are not case-sensitive
disposition-parm := filename-parm / parameter
filename-parm := "filename" "=" value;
Content-Disposition属性有两种类型：inline 和 attachment inline ：将文件内容直接显示在页面 attachment：弹出对话框让用户下载具体例子：
Content-Type: image/jpeg
Content-Disposition: inline;filename=hello.jpg
Content-Description: just a small picture of me
在页面内打开代码：
File file = new File("rfc1806.txt");
String filename = file.getName();
response.setHeader("Content-Type","text/plain");
response.addHeader("Content-Disposition","inline;filename=" + new String(filename.getBytes(),"utf-8"));
response.addHeader("Content-Length","" + file.length());
弹出保存框代码：
File file = new File("rfc1806.txt");
String filename = file.getName();
response.setHeader("Content-Type","text/plain");
response.addHeader("Content-Disposition","attachment;filename=" + new String(filename.getBytes(),"utf-8"));
response.addHeader("Content-Length","" + file.length());
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