Downloadable ZIP files displaying to screen

-- url of the page with the problem -- : www.rmisos.net/shop-downloads
-- HikaShop version -- : 2.3.3
-- Joomla version -- : 3.2.3
-- PHP version -- : 5.4.33
-- Browser(s) name and version -- : Firefox 26

After upgrading from version 2.2.2 to 2.3.3 the downloads are broken. The headers are getting sent before the download headers get sent and so the browser thinks it is getting HTML rather than a ZIP file.

Using the Firefox Live HTTP headers plugin I see this:
www.rmisos.net/shop-downloads/order/down...le_id-64/order_id-54

GET /shop-downloads/order/download/file_id-64/order_id-54 HTTP/1.1
Host: www.rmisos.net
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: www.rmisos.net/shop-downloads
Cookie: SERVERID=elite1; b77c27571174501cd00dfb7f6ee4ce65=959tm1e2c7okfmktfmj8v7uuo3
Connection: keep-alive

HTTP/1.1 200 OK
Date: Wed, 01 Oct 2014 10:56:07 GMT
Server: Apache/2.2.15 (CentOS)
X-Powered-By: PHP/5.4.33
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8

Looking at the Apache error log, I can see this:
[Wed Oct 01 12:49:40 2014] [error] [client 91.186.19.66] PHP Warning: Cannot modify header information - headers already sent by (output started at /var/www/vhosts/rmisos.net/httpdocs/administrator/components/com_hikashop/helpers/helper.php:1) in /var/www/vhosts/rmisos.net/httpdocs/administrator/components/com_hikashop/classes/file.php on line 483, referer: www.rmisos.net/shop-downloads

It says the header output was started in administrator/components/com_hikashop/helpers/helper.php, but unfortunately it doesn't tell me the line number.

Can you help me track this down? I'm sending you a login via email so you can see for yourselves.

Thanks for response.

I converted the file administrator/components/com_hikashop/helpers/helper.php to be UTF8 without BOM and now downloads work again.

Problem solved!