Upload a text file as attachment using Bot framework not working in Skype channel

I am unable to upload a text file as an attachment using the Bot framework (Bot.Builder v3.11.0), when trying in the Skype channel. It worked in the Bot framework emulator though. Following is the code which uploads a file and to returns the activity with the uploaded file URL in the attachment. It throws an exception when using the Skype channel. Alternatively is there any other way to achieve uploading/attaching a text file in the Skype channel which the user can then download from within the client?

public static async Task<Activity> GetTextAttachmentAsync(Activity message)
{
    var reply = message.CreateReply("Here is a text attachment");
    var serviceUrl = reply.ServiceUrl;
    var conversationId = reply.Conversation.Id;

    byte[] fileData = null;
    using (var wc = new System.Net.WebClient())
        fileData = wc.DownloadData("https://textfiles.com/100/adventur.txt");

    using (var connector = new ConnectorClient(new Uri(serviceUrl)))
    {
        var attachments = new Attachments(connector);
        var token = await (connector.Credentials as MicrosoftAppCredentials).GetTokenAsync();
        connector.HttpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
        var response = await attachments.Client.Conversations.UploadAttachmentAsync(
            conversationId,
            new AttachmentData
            {
                Name = "transcript.html",
                OriginalBase64 = fileData,
                Type = "text/html"
            });

        reply.Attachments = new List<Attachment>
        {
            new Attachment
            {
                Name = "transcript.html",
                ContentType = "text/html",
                ContentUrl = attachments.GetAttachmentUri(response.Id)
            }
        };

        return reply;
    }
}

Exception thrown from the UploadAttachmentAsync() function above:

Microsoft.Rest.HttpOperationException: Not Found
   at Microsoft.Bot.Connector.ErrorHandling.<HandleErrorAsync>d__2`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.Bot.Connector.ConversationsExtensions.<UploadAttachmentAsync>d__15.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Support.Services.Bot.Core.Utilities.AdaptiveCardsHelper.<GetTextAttachmentAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Support.Services.Bot.Core.Dialogs.BotDialog.<HandleMessageAsync>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Support.Services.Bot.Core.Dialogs.DialogBase`1.<MessageReceivedAsync>d__8.MoveNext()

When running your code with Common Language Runtime Errors enabled in Visual Studio. I got the following error:

It seems that the site address https://textfiles.com/100/adventur.txt does not have a trusted certificate and that makes .NET unhappy. When I went to investigate, I found this Stack Overflow answer that suggested using this code below to overcome this but is strongly recommended not to use this in production.

ServicePointManager.ServerCertificateValidationCallback += (o, c, ch, er) => true;

This was going to be my original suggestion before I ran your code: many channels (Skype included), limit the types of files you are able to send from a bot when using base64 and/or local files. For example, I know that you cannot send PDF files as Base64 in Skype. If memory serves correctly, you can only send image and video files (maybe audio too) using the base64 method in Skype. So even if you resolve this error you may run into this afterward. The workaround for this is using hosted files. I'm not exactly sure what you are trying to do with your bot, so I'm not sure if this is an option for you specifically, but it is an option.

So if you find this code not working after resolving the certificate issue, try sending an image file and see if that works, if it does but your HTML file still fails, you will know this is why.