FTPS download and upload a file

ftps.fs

// FTPS = SSL encrypted FTP.
// SFTP = FTP over SSH. 

// Set this as config, as the server may change the certificate.
let ``trusted server certificate hash`` = "603780E732DB12D0F6BA434BA8E04D141904A165"

open System
open System.IO
open System.Net
open System.Net.Security

type FTPSClient(username:string, password:string) =
    inherit WebClient(Credentials =
        (NetworkCredential(username.Normalize(), password.Normalize()) :> ICredentials),
         UseDefaultCredentials = false)
    override __.GetWebRequest(address) =
        let request = base.GetWebRequest(address) :?> FtpWebRequest
        request.Credentials <- NetworkCredential(username.Normalize(), password.Normalize())
        request.EnableSsl <- true
        request.UsePassive <- true
        //request.UseBinary <- t
        ServicePointManager.ServerCertificateValidationCallback <-
            RemoteCertificateValidationCallback(
                fun obj serverCert chain errors ->
                    // Validate server security certificate here!
                    if serverCert.GetCertHashString() = ``trusted server certificate hash`` then
                        true
                    else
                    match errors with
                    | SslPolicyErrors.None -> true
                    | x -> Console.WriteLine("Server SSL invalid: " +
                                Enum.GetName(typeof<SslPolicyErrors>, (box x)))
                           false)
        let cert =
            request.ClientCertificates.Add(
                new System.Security.Cryptography.X509Certificates.X509Certificate())
        request :> WebRequest

let downloadFTPS host username (password:string) fileName targetPath =
    async {
        use client = new FTPSClient(username, password)
        let fileToGet = Uri("ftp://" + host + "/" + fileName)
        let target = Path.Combine [| targetPath; fileName |]
        return! client.DownloadFileTaskAsync(fileToGet, target) |> Async.AwaitTask
    }

let uploadFTPS host username (password:string) filePath =
    async {
        let fileName = Path.GetFileName(filePath)
        use client = new FTPSClient(username, password)
        let fileToSend = Uri("ftp://" + host + "/" + fileName)
        return!
            client.UploadFileTaskAsync(fileToSend, WebRequestMethods.Ftp.UploadFile, filePath)
            |> Async.AwaitTask |> Async.Catch
    }

//downloadFTPS "test.rebex.net" "demo" "password" "readme.txt" @"c:\temp\"
//|> Async.RunSynchronously

//uploadFTPS "test.rebex.net" "demo" "password" @"C:\Temp\test3.txt"
//|> Async.RunSynchronously