"jpeg_quality" filter not working.

As a test, I wanted to set the image quality of uploaded images to 50%, so I used the “jpeg_quality” filter with this code:

function compress_jpeg( $quality ) {
	return (int)50;
}
add_filter( 'jpeg_quality', 'compress_jpeg' );

Unfortunately, no image compression seems to be taking place. Does anyone have an idea on why this isn’t working?