PHP下载停止在99%下载管理器

我有一个存放文件的服务器，人们可以下载这些文件。我使用以下代码发送下载:

<?php
function dl_file_resumable($file, $is_resume = true)
{
    //First, see if the file exists
    if (!is_file($file)) {
        header("HTTP/1.1 400 Invalid Request");
        die("<h3>File Not Found</h3>");
    }
    //Gather relevent info about file
    $size = filesize($file);
    $fileinfo = pathinfo($file);
    //workaround for IE filename bug with multiple periods / multiple dots in filename
    //that adds square brackets to filename - eg. setup.abc.exe becomes setup[1].abc.exe
    $filename = (isset($_SERVER['HTTP_USER_AGENT']) && strstr($_SERVER['HTTP_USER_AGENT'], 'MSIE')) ?
                  preg_replace('/'./', '%2e', $fileinfo['basename'], substr_count($fileinfo['basename'], '.') - 1) :
                  $fileinfo['basename'];
    $file_extension = strtolower($fileinfo['extension']);
    //This will set the Content-Type to the appropriate setting for the file
    require 'mineTypes.php';
    // Check if the file extension is in $ctype_array & return the value. If not, send default.
    $ctype = array_key_exists($file_extension, $ctype_array) ? $ctype_array[$file_extension] : 'application/force-download';
    $is_resume = ($is_resume && isset($_SERVER['HTTP_RANGE']));
    //check if http_range is sent by browser (or download manager)
    if($is_resume)
    {
        $arr = explode('=', $_SERVER['HTTP_RANGE'], 2);
        if(isset($arr[1]))   
            list($size_unit, $range_orig) = $arr;
        else list($size_unit) = $arr;
        if ($size_unit == 'bytes')
        {
            //multiple ranges could be specified at the same time, but for simplicity only serve the first range
            //http://tools.ietf.org/id/draft-ietf-http-range-retrieval-00.txt
            $arr3 = explode(',', $range_orig, 2);
            if(isset($arr3[1]))
                list($range, $extra_ranges) = $arr3;
            else
                list($range) = $arr3;
        }
        else
        {
            $range = '';
        }
    }
    else
    {
        $range = '';
    }
    //figure out download piece from range (if set)
    $arr2 = explode('-', $range, 2);
    if(isset($arr2[1]))
        list($seek_start, $seek_end) = $arr2;
    else
        list($seek_start) = $arr2;
    //set start and end based on range (if set), else set defaults
    //also check for invalid ranges.
    $seek_end = (empty($seek_end)) ? ($size - 1) : min(abs(intval($seek_end)),($size - 1));
    $seek_start = (empty($seek_start) || $seek_end < abs(intval($seek_start))) ? 0 : max(abs(intval($seek_start)),0);
    //add headers if resumable
    if ($is_resume)
    {
        //Only send partial content header if downloading a piece of the file (IE workaround)
        if ($seek_start > 0 || $seek_end < ($size - 1))
        {
            header('HTTP/1.1 206 Partial Content');
        }
        header('Accept-Ranges: bytes');
        header('Content-Range: bytes '.$seek_start.'-'.$seek_end.'/'.$size);
    }
    header('Content-Type: ' . $ctype);
    header('Content-Length: '.($seek_end - $seek_start + 1));
    header('Content-Disposition: attachment; filename="' . $filename . '"');
    header('Accept-Ranges: bytes');
    //reset time limit for big files
    set_time_limit(0);
    //open the file
    $fp = fopen($file, 'rb');
    //seek to start of missing part
    fseek($fp, $seek_start);
    //start buffered download
    $bytes = 0;
    while(!feof($fp))
    {
        print(fread($fp, 1024*8));
        flush();
        $bytes = $bytes + 1024 * 8;
        ob_flush();        
    }
    fclose($fp);
    exit;
}

当我通过浏览器下载文件时一切正常，但当我通过下载管理器(如IDM)下载时-下载在99%时停止。

怎么了?