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

It’s possible to add extensions to existing Swift object types using extensions, as described in the language specification.

As a result, it’s possible to create extensions such as:

extension String {
    var utf8data:NSData {
        return self.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
    }
}

However, what’s the best naming practice for Swift source files containing such extensions?

In the past, the convention was to use extendedtype+categoryname.m for the Objective-C
type as discussed in the Objective-C guide. But the Swift example doesn’t have a category name, and calling it String.swift doesn’t seem appropriate.

So the question is: given the above String extension, what should the swift source file be called?