'.../core/file_api.php' line 917
Probably not version 1.2.11 of MantisBT
Number of lines of file_api.php is 910

Can you try replacing function file_get_content_type_override in file_api.php by the following code

function file_get_content_type_override( $p_filename ) {

global $g_file_download_content_type_overrides;

$t_extension = pathinfo( $p_filename, PATHINFO_EXTENSION );

if ( isset ( $g_file_download_content_type_overrides[$t_extension] ) ) {
    return $g_file_download_content_type_overrides[$t_extension];
}
return false;

}

One more question:
Did you change default settings of $g_display_errors ?

Dear atrol,

This additional "if" statement solves the problem, thanks a lot.

For testing purposes, I have set:
$g_display_errors = array(
E_WARNING => 'inline',
E_NOTICE => 'inline',
E_USER_ERROR => 'halt',
E_USER_WARNING => 'inline',
E_USER_NOTICE => 'inline'
);

Kind regards,
awerner

Marking as 'acknowledged' not resolved/closed to track that change gets ported to master-2.0.x branch