Webp images are not allowed to WordPress media library with default setting. To enable the .webp format use the following code block in your functions.php file inside the theme folder.

/* Enable webp image support for media */
function webp_upload_mimes( $existing_mimes ) {
	// add webp to the list of mime types
	$existing_mimes['webp'] = 'image/webp';

	// return the array back to the function with our added mime type
	return $existing_mimes;
}
add_filter( 'mime_types', 'webp_upload_mimes' );

/* Enable preview / thumbnail for webp image files. */
function webp_is_displayable($result, $path) {
    if ($result === false) {
        $displayable_image_types = array( IMAGETYPE_WEBP );
        $info = @getimagesize( $path );

        if (empty($info)) {
            $result = false;
        } elseif (!in_array($info[2], $displayable_image_types)) {
            $result = false;
        } else {
            $result = true;
        }
    }

    return $result;
}
add_filter('file_is_displayable_image', 'webp_is_displayable', 10, 2);