Xamarin.DropBox.APi - HTTP 400 (Bad request) on DownloadAsync

Hi,

I have the following code in a static helper class which works perfectly in a Windows UWP app:

public static DropboxClient dbxClient { get; private set; }

......

public static async Task<string> DownloadFile(string filePath, string fileName, string accessToken)
{
            string output = null;

            try
            {
                if (dbxClient == null)
                    dbxClient = new DropboxClient(accessToken);

                if (dbxClient != null)
                {
                    var downloadResponse = await dbxClient.Files.DownloadAsync(filePath + fileName);
                    var content = await downloadResponse.GetContentAsStringAsync();
                    DebugInfo += $"{filePath} downloaded - revision {downloadResponse.Response.Rev}";
                    output = content;
                }
                else
                    throw new Exception("DownloadFile: DropBox client not authenticated");

            }
            catch (Exception ex)
            {
                Exception = ex;
                output = ex.Message;
            }

            return output;
        }

Now I want to convert the app I created in UWP to Android  so I copied it into my new Xamarin.Forms app and replaced the usual Dropbox.API NuGet package with Xamarin.DropBox.Api, but when I run the code I get this message:

Error in call to API function "files/download": Bad HTTP "Content-Type"
header: "application/x-www-form-urlencoded".  Expecting one of "text/plain", "text/plain; charset=utf-8", "application/octet-stream",
"application/octet-stream; charset=utf-8".

I have googled this and searched on this forum but (so far) can't find anything relevant - can someone please let me know what I'm doing wrong?  DropBoxClient seems to want a content-type (which is fair enough) but there does not seem to have any way to actually specify that from what I can see in the DropBoxClient properties and methods available.

 Thanks in advance for any help offered.