file(extensions:)
The completion candidates include directory and file names, the latter filtered by the given list of extensions.
static func file(extensions: [String] = []) -> CompletionKind
If the given list of extensions is empty, then file names are not filtered.
Given file extensions must not include the .
initial extension separator.
Given file extensions are parsed by the requesting shell as globs; Swift Argument Parser does not perform any escaping or quoting.
The directory/file filter and the given list of extensions are included in a completion script when it is generated.