Source

File: wp-includes/class-wp-image-editor.php

protected static function get_mime_type( $extension = null ) {
		if ( ! $extension ) {
			return false;
		}

		$mime_types = wp_get_mime_types();
		$extensions = array_keys( $mime_types );

		foreach ( $extensions as $_extension ) {
			if ( preg_match( "/{$extension}/i", $_extension ) ) {
				return $mime_types[ $_extension ];
			}
		}

		return false;
	}