"Restream" images to Azure Blob Storage from Web API

AutoGeneratingImageNameAzureBlobStorageMultipartProvider

 public class AutoGeneratingImageNameAzureBlobStorageMultipartProvider : MultipartStreamProvider
{
    /* Let the consumer read the generated filenames and the headers after uploading for persisting to database etc. */
    public Dictionary<string, HttpHeaders> UploadedFiles { get; set; }
    
    private readonly bool _returnFullUri;
    private readonly string _blockprefix;
    private readonly CloudBlobContainer _container;

    protected Dictionary<string,string> ImageMediaTypesToExtensions = new Dictionary<string, string>()
    {
        {"image/gif", ".gif"},
        {"image/jpeg", ".jpg"},
        {"image/png", ".png"}
    };

    public AutoGeneratingImageNameAzureBlobStorageMultipartProvider(CloudBlobContainer container, string blockprefix = "", bool returnFullUri = true)
    {
        _blockprefix = blockprefix;
        _container = container;
        _returnFullUri = returnFullUri;
        UploadedFiles = new Dictionary<string, HttpHeaders>();
    }
    
    /* According to documentation the caller is responsible for closing the returned stream */
    public override Stream GetStream(HttpContent parent, HttpContentHeaders headers)
    {
        
        string mediatype = headers.ContentType.MediaType.ToLower();
        string extension;
        if (!ImageMediaTypesToExtensions.TryGetValue(mediatype, out extension))
        {
            throw new FormatException(string.Format("Unknown image mediatype: '{0}' for file with original name '{1}'", mediatype, headers.ContentDisposition.FileName));
        }
        string filename = string.Concat(Guid.NewGuid().ToString(), extension);
        CloudBlockBlob blob = _container.GetBlockBlobReference(string.Concat(_blockprefix, filename));
        UploadedFiles.Add(_returnFullUri ? blob.Uri.AbsoluteUri : filename, headers);
        return blob.OpenWrite();
    }
  }