# fileNameExtension(forType:saveOperation:) Returns a file extension to append to the file URL of the document file being written. ``` func fileNameExtension(forType typeName: String?, saveOperation: UIDocument.SaveOperation) -> String ``` ## Parameters `typeName` A Uniform Type Identifier (UTI) that indicates the type of document (for example, PDF or HTML). `saveOperation` A constant that indicates whether the document file is being written the first time or whether it’s being overwritten. See [`UIDocument.SaveOperation`](/documentation/UIKit/UIDocument/SaveOperation) for details. ## Return Value A string to use as the file extension of the document file. ## Discussion The default implementation queries Launch Services to obtain the file extension matching the file (document) type. You can override this method to return a file extension that’s different from the default extension. The default implementation of the [`save(to:for:completionHandler:)`](/documentation/UIKit/UIDocument/save(to:for:completionHandler:)) method calls this method before it gets the document content and writes the document file to disk. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)