if (message.Attachments != null && message.Attachments.Any())
{
    var attachment = message.Attachments.First();
    using (HttpClient httpClient = new HttpClient())
    {
        if ((message.ChannelId.Equals("skype", StringComparison.InvariantCultureIgnoreCase) || message.ChannelId.Equals("msteams", StringComparison.InvariantCultureIgnoreCase)) && new Uri(attachment.ContentUrl).Host.EndsWith("skype.com"))
        {
            var token = await new MicrosoftAppCredentials().GetTokenAsync();
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
        }

        var responseMessage = await httpClient.GetAsync(attachment.ContentUrl);
        var contentLenghtBytes = responseMessage.Content.Headers.ContentLength; // this is populated correctly

        if(attachment.Name.ToLower().Equals("opportunity.xlsx"))
        {
            CreateOpportunity(context, attachment.Content); // This Content is always null, even though everything else is populated.
        }
    }
}