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Dealing with file extensions and Uniform Type Identifiers

Published on · Updated on
1 minute read

There are different ways of representing a type of file, two popular ones being file extensions and another being Uniform Type Identifiers (UTIs) from the UIKit APIs. When working with a UIKit API such as UIDocumentPickerViewController we may need to distinguish between these types.

We can create independent types to hold file extensions and type identifiers, this makes it really clear which one is required for a particular piece of functionality.

struct FileExtension {
let rawValue: String

func asTypeIdentifier() -> TypeIdentifier {
let identifierCreated = UTTypeCreatePreferredIdentifierForTag(
kUTTagClassFilenameExtension, rawValue as NSString, nil
if let typeIdentifier = identifierCreated?.takeRetainedValue() {
return TypeIdentifier(rawValue: typeIdentifier as String)
return TypeIdentifier(rawValue: "")

struct TypeIdentifier {
let rawValue: String

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Hi, I hope you enjoyed the article. I am Andrew - a builder of apps and developer tools. Articles on the blog focus on all aspects of Android and iOS development using Kotlin and Swift.

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