Source: iamnag.com

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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);