NGINX X-Accel-Redirect doesn't pass Content-Type automatically?

Question

Doesn't (or can't) NGINX's X-Accel-Redirect automatically pass the Content-Type header based on the file?

The NGINX part:

location /file {
    rewrite /file/(.+) /file.php?params=$1 last;
}

location ~ /files {
    internal;
    root /path/to/files;
}

PHP:

$filename = '/files/test.png';
header('Content-Disposition: inline; filename = ' . $filename);
header('X-Accel-Redirect: ' . $filename);

I have to add header('Content-type: image/png'); to display the png directly, otherwise the server will return text/html

Isn't there a way to get the content type directly from NGINX? Or is this a bad idea (perhaps performance)? I could store the content type in the database with all the other stuff, that's not problem. Just wonder why it doesn't work.

Looks like this helps...

header('Content-type:');

but why?

Thanks!