$g_allow_file_cache (if defined) triggers emission of a Pragma: no-cache header, which is not only a deprecated HTTP/1.0 thing that should be replaced with a Cache-Control header, but also an incorrect usage of the directive which is only defined in a request context (not response).

The pragma was added to fix an issue downloading large attachments - it is quite likely that this code is no longer useful nowadays - it was written in 2005.

mantisbt/file_download.php Lines 154 to 164 in 262ecdd

 # To fix an IE bug which causes problems when downloading 
 # attached files via HTTPS, we disable the "Pragma: no-cache" 
 # command when IE is used over HTTPS. 
 global $g_allow_file_cache; 
 if( http_is_protocol_https() && is_browser_internet_explorer() ) { 
    # Suppress "Pragma: no-cache" header. 
 } else { 
    if( !isset( $g_allow_file_cache ) ) { 
        header( 'Pragma: no-cache' ); 
    } 
 } 

We can probably get rid of the whole thing.