saving file to SD-card for public access

How in Xamarin.Android save the file to an external source (SD card)?
There is a task to save the file so that the user can then work with this file for review and outside the application.
The code below is working, to save the file in a specialized directory. But this directory is hidden from the user and you can not work with these files.

public class FileWorker_Android : IFileWorker
    {
        public Task DeleteAsync(string filename)
        {
            File.Delete(GetFilePath(filename));
            return Task.FromResult(true);
        }

        public Task<bool> ExistsAsync(string filename)
        {
            string filepath = GetFilePath(filename);
            // существует ли файл
            bool exists = File.Exists(filepath);
            return Task<bool>.FromResult(exists);
        }

        public Task<IEnumerable<string>> GetFilesAsync()
        {
            IEnumerable<string> filenames = from filepath in Directory.EnumerateFiles(GetDocsPath())
                                            select Path.GetFileName(filepath);
            return Task<IEnumerable<string>>.FromResult(filenames);
        }

        public async Task<string> LoadTextAsync(string filename)
        {
            string filepath = GetFilePath(filename);
            using (StreamReader reader = File.OpenText(filepath))
            {
                return await reader.ReadToEndAsync();
            }
        }

        public async Task SaveTextAsync(string filename, string text)
        {
            string filepath = GetFilePath(filename);
            using (StreamWriter writer = File.CreateText(filepath))
            {
                await writer.WriteAsync(text);
            }
        }
        string GetFilePath(string filename)
        {
            return Path.Combine(GetDocsPath(), filename);
        }
        // Directory MyDocuments
        string GetDocsPath()
        {
            return Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
        }
    }

From the Android documentation, all work with an external source (SD card) occurs via Environment.getExternalStoragePublicDirectory, but in Xamarin in Environment there is no such method