private String getExtra(String extraFilename){		
		File extraFile = new File(extraFilename);
		
		if(!extraFile.exists()){
			return "";
		}
		String name = extraFilename.substring(extraFilename.lastIndexOf("\\")+1);//ļ
		String extName = name.substring(name.lastIndexOf(".")+1);//չ
		StringBuffer declaration = new StringBuffer();//
		StringBuffer content = new StringBuffer();//

		FileInputStream fis = null;
		try {
			fis = new FileInputStream(extraFile);			
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}
		byte[] b = new byte[1024];
		try {
			while(fis.read(b)!=-1){
				content.append(encoder.encode(b));
			}			
		} catch (IOException e) {
			e.printStackTrace();
		}
		//
		declaration.append("Content-Type: ");
		if("rar".equals(extName)){//rar
			declaration.append("application/x-rar-compressed;\n");
		}else if("txt".equals(extName)||"exe".equals(extName)){//txt,exe
			declaration.append("application/octet-stream;\n");
		}else if("gif".equals(extName)||"jpeg".equals(extName)){//gif,jpeg
			declaration.append("image/"+extName+";\n");
		}else if("mp3".equals(extName)){
			declaration.append("audio/x-mpeg;\n");
		}else if("ppt".equals(extName)){
			declaration.append("application/vnd.ms-powerpoint;\n");
		}else if("rmvb".equals(extName)||"rm".equals(extName)){
			declaration.append("audio/x-pn-realaudio;\n");
		}else if("wav".equals(extName)){
			declaration.append("audio/x-wav;\n");
		}else if("doc".equals(extName)){
			declaration.append("application/msword;\n");
		}
		declaration.append("\tcharset=\"gbk\";\n");
		declaration.append("\tname=\""+getPreAndSuf(name)+"\"\n");
		declaration.append("Content-Disposition: attachment; filename=\""+
				getPreAndSuf(name)+"\"\n");
		declaration.append("Content-Transfer-Encoding: base64");
		
		return declaration+"\n\n"+content.toString();		
	}