WordPress at a glance
filter is not described

wp_mail_content_type filter-hook . WP 2.3.0

Filters the wp_mail() content type.

Usage

add_filter( 'wp_mail_content_type', 'filter_function_name_3574' );
function filter_function_name_3574( $content_type ){
	// filter...

	return $content_type;
}
$content_type(string)
Default wp_mail() content type.

Where the hook is called

wp_mail()
wp_mail_content_type
wp_staticize_emoji_for_email()
wp_mail_content_type
wp-includes/pluggable.php 427
$content_type = apply_filters( 'wp_mail_content_type', $content_type );
wp-includes/formatting.php 5292
$content_type = apply_filters( 'wp_mail_content_type', $content_type );

Where the hook is used (in WP core)

Does not used.