What's the best practice for naming Swift files that add extensions to existing objects?

Most examples I have seen mimic the Objective-C approach. The example extension above would be:

String+UTF8Data.swift

The advantages are that the naming convention makes it easy to understand that it is an extension, and which Class is being extended.

The problem with using Extensions.swift or even StringExtensions.swift is that it's not possible to infer the purpose of the file by its name without looking at its contents.

Using xxxable.swift approach as used by Java works okay for protocols or extensions that only define methods. But again, the example above defines an attribute so that UTF8Dataable.swift doesn't make much grammatical sense.

There is no Swift convention. Keep it simple:

StringExtensions.swift

I create one file for each class I'm extending. If you use a single file for all extensions, it will quickly become a jungle.

I prefer having a + to underline the fact it contains extensions :

String+Extensions.swift

And if the file gets too big, you can then split it for each purpose :

String+UTF8Data.swift

String+Encrypt.swift