Source

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

public static function supports_mime_type( $mime_type ) {
		$image_types = imagetypes();
		switch ( $mime_type ) {
			case 'image/jpeg':
				return ( $image_types & IMG_JPG ) != 0;
			case 'image/png':
				return ( $image_types & IMG_PNG ) != 0;
			case 'image/gif':
				return ( $image_types & IMG_GIF ) != 0;
		}

		return false;
	}