PHP 文件下载，下载后的文件和源文件内容不一样

新手自学，部分代码如下：
html代码：

 <form action="download_file.php" method="get">

            <input  name="filename" type="text" style="width: 17%;" value="" placeholder="请输入文件名，英文区分大小写" />

            <input type="submit" value="搜索" />

        </form>

download_file.php代码：

$file_name = $_GET['filename'];

    $file_dir="upload/";

        $file=fopen($file_dir.$file_name,"r");//打开文件
        //echo($file_dir.$file_name);
        Header("Content-type: application/octet-stream");  
         Header("Accept-Ranges:bytes");  
        Header("Accept-Length:".filesize($file_dir.$file_name));  
        Header("Content-Disposition: attachment; filename=".$file_name);
        readfile($file_dir.$file_name);

比如说我下载一个.txt的文件，执行文件后，得到的.txt文件里的内容和源文件完全不一样；

另一个问题是在搜索栏里如何只搜索文件名就能下载，而不用输入文件名+后缀，
请教大神代码解答！

<?php
/**

• CodeIgniter *
• An open source application development framework for PHP *
• This content is released under the MIT License (MIT) *
• Copyright (c) 2014 - 2017, British Columbia Institute of Technology *
• Permission is hereby granted, free of charge, to any person obtaining a copy
• of this software and associated documentation files (the "Software"), to deal
• in the Software without restriction, including without limitation the rights
• to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
• copies of the Software, and to permit persons to whom the Software is
• furnished to do so, subject to the following conditions: *
• The above copyright notice and this permission notice shall be included in
• all copies or substantial portions of the Software. *
• THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
• IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
• FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
• AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
• LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
• OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
• THE SOFTWARE. *
• @package CodeIgniter
• @author EllisLab Dev Team
• @copyright Copyright (c) 2008 - 2014, EllisLab, Inc. (https://ellislab.com/)
• @copyright Copyright (c) 2014 - 2017, British Columbia Institute of Technology (http://bcit.ca/)
• @license http://opensource.org/licenses/MIT MIT License
• @link https://codeigniter.com
• @since Version 1.0.0
• @filesource */ defined('BASEPATH') OR exit('No direct script access allowed');

/**

• CodeIgniter Download Helpers *
• @package CodeIgniter
• @subpackage Helpers
• @category Helpers
• @author EllisLab Dev Team
• @link https://codeigniter.com/user_guide/helpers/download_helper.html */

// ------------------------------------------------------------------------

if ( ! function_exists('force_download'))
{
/**
* Force Download
*
* Generates headers that force a download to happen
*
* @param string filename
* @param mixed the data to be downloaded
* @param bool whether to try and send the actual file MIME type
* @return void
*/
function force_download($filename = '', $data = '', $set_mime = FALSE)
{
if ($filename === '' OR $data === '')
{
return;
}
elseif ($data === NULL)
{
if ( ! @is_file($filename) OR ($filesize = @filesize($filename)) === FALSE)
{
return;
}

        $filepath = $filename;
        $filename = explode('/', str_replace(DIRECTORY_SEPARATOR, '/', $filename));
        $filename = end($filename);
    }
    else
    {
        $filesize = strlen($data);
    }

    // Set the default MIME type to send
    $mime = 'application/octet-stream';

    $x = explode('.', $filename);
    $extension = end($x);

    if ($set_mime === TRUE)
    {
        if (count($x) === 1 OR $extension === '')
        {
            /* If we're going to detect the MIME type,
             * we'll need a file extension.
             */
            return;
        }

        // Load the mime types
        $mimes =& get_mimes();

        // Only change the default MIME if we can find one
        if (isset($mimes[$extension]))
        {
            $mime = is_array($mimes[$extension]) ? $mimes[$extension][0] : $mimes[$extension];
        }
    }

    /* It was reported that browsers on Android 2.1 (and possibly older as well)
     * need to have the filename extension upper-cased in order to be able to
     * download it.
     *
     * Reference: http://digiblog.de/2011/04/19/android-and-the-download-file-headers/
     */
    if (count($x) !== 1 && isset($_SERVER['HTTP_USER_AGENT']) && preg_match('/Android\s(1|2\.[01])/', $_SERVER['HTTP_USER_AGENT']))
    {
        $x[count($x) - 1] = strtoupper($extension);
        $filename = implode('.', $x);
    }

    if ($data === NULL && ($fp = @fopen($filepath, 'rb')) === FALSE)
    {
        return;
    }

    // Clean output buffer
    if (ob_get_level() !== 0 && @ob_end_clean() === FALSE)
    {
        @ob_clean();
    }

    // Generate the server headers
    header('Content-Type: '.$mime);
    header('Content-Disposition: attachment; filename="'.$filename.'"');
    header('Expires: 0');
    header('Content-Transfer-Encoding: binary');
    header('Content-Length: '.$filesize);
    header('Cache-Control: private, no-transform, no-store, must-revalidate');

    // If we have raw data - just dump it
    if ($data !== NULL)
    {
        exit($data);
    }

    // Flush 1MB chunks of data
    while ( ! feof($fp) && ($data = fread($fp, 1048576)) !== FALSE)
    {
        echo $data;
    }

    fclose($fp);
    exit;
}

}

不是说的很清楚了，没有这个文件，已经报错了

readfile怎么写的？