According to RFC-6266(4.3. Disposition Parameter: 'Filename'), 'filename' param on 

Content-Disposition in HTTP header should be encode.

But current of lib/filelib.php is not compatible on RFC-6266.

For example, lib/filelib.php on line 2290, urlencoded only if IE. But, we should urlencode if ANY BROWSERS.

As a result, when download file named by multi-byte(ex. サンプルファイル.zip), then garbled on some of WAF and Load Balancer implemented on RFC-6266.

(Using Chrome, Firefox)

In my case, this problem occurred on Azure Application Gateway .

Bit this problem will occurred on another WAF or LoadBalancer implemented on RFC-6266.

I want to suggest "delete if (core_useragent::is_ie()) { on urlencode to urlencode on any browsers."

if (core_useragent::is_ie()) {
    $filename = urlencode($filename);
}

to 

$filename = urlencode($filename);

on lib/filelib.php line 2263.