Remove Pragma headers in file_download.php

[dregad]

Fixes #33007

[atrol]

As this is the only place where $g_allow_file_cache is used, you can also remove it from config_defaults_inc.php and the documentation.

[vboctor]

I was working on the caching to leverage the fact that attachments are never updated and came up with the following which reduces requests to server and works really well:

$t_last_modified_time = $v_date_added;
$t_etag = md5( $t_last_modified_time );

header( 'Cache-Control: private, max-age=31536000' );
header( 'Expires: ' . gmdate( 'D, d M Y H:i:s \G\M\T', time() + 31536000 ) );
header( 'ETag: ' . $t_etag );
header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s \G\M\T', $t_last_modified_time ) );
header( 'Pragma: cache' );

# This is needed to avoid Mantis preparing the payload and then the http server not sending it to the client
if ( ( isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) && trim( $_SERVER['HTTP_IF_NONE_MATCH'] ) == $t_etag ) ||
     ( isset( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) && strtotime( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) == strtotime( $t_last_modified_time ) ) ) {
    header('HTTP/1.1 304 Not Modified');
    exit;
}

[dregad]

As this is the only place where $g_allow_file_cache is used, you can also remove it from config_defaults_inc.php and the documentation.

@atrol done

[dregad]

I was working on the caching to leverage the fact that attachments are never updated and came up with the following which reduces requests to server and works really well:

@vboctor I'm fine with that although I did not actually test it, but this change is outside of this PR's scope so please open a new issue on the tracker and a separate PR with your proposed fix.

[vboctor]

@dregad I just did some testing, it works well for the download scenario, but not the preview scenario, since a unique security token is added to the preview URLs, which cause it not to cache. For the preview scenario to work, we will have to have an alternative for file_show_inline_token query string.

I agree it is outside the scope of this PR.

[dregad]

@atrol without feedback from you since I pushed it last week, I assume you're OK with the commit removing allow_file_cache config.