Don't forget to force unwrap optionals when putting them into dictionaries which will be used not on iOS

gistfile1.swift

func unauthenticatedUserSessionConfiguration() -> NSURLSessionConfiguration {
    let base64Credentials = ("\(clientID):\(clientSecret)" as NSString).dataUsingEncoding(NSUTF8StringEncoding)
    let encoded = base64Credentials?.base64EncodedDataWithOptions(NSDataBase64EncodingOptions(rawValue: 0))
        
    var headers = Alamofire.Manager.sharedInstance.session.configuration.HTTPAdditionalHeaders ?? [:]
    headers["X-2FA-Support"] = "default"
        
        
// This is the key line. Note that it's an optional string.
    headers["Authorization"] = "Basic \(NSString(data: encoded!, encoding: NSUTF8StringEncoding))"

// The value is: "Basic Optional(MzA5MWY1234mU1234kYTJkOTM212I3ZWI4YzE2NACkMDY4YmEw88E3ZTo3ZD1234IxYzE481234DM5JKdkNTZLMAO0MTNkMmEyNDJhYzIxNGIz)"
// And your request will fail.
        
// But if we force unwrap the optional as such.
    headers["Authorization"] = "Basic \(NSString(data: encoded!, encoding: NSUTF8StringEncoding)!)"

// The value is: "Basic MzA5MWY1234mU1234kYTJkOTM212I3ZWI4YzE2NACkMDY4YmEw88E3ZTo3ZD1234IxYzE481234DM5JKdkNTZLMAO0MTNkMmEyNDJhYzIxNGIz"
// And your request will succeed!
        
    let configuration = NSURLSessionConfiguration.ephemeralSessionConfiguration()
    configuration.HTTPAdditionalHeaders = headers
        
    return configuration
}