将png文件上传到Azure Blob Storage会失去透明度

我们很难弄清楚如何上传png文件到Azure Blob Storage而不丢失图像透明度。

我们正在使用Azure的存储SDK，代码如下:

var storageAccount = CloudStorageAccount.Parse(*blob_connection_string*);
//create client to work with blobs
CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
//already created container via azure management portal, set container reference
CloudBlobContainer container = blobClient.GetContainerReference("brand");
foreach (string file in httpRequest.Files)
{
    var postedFile = httpRequest.Files[file];
    CloudBlockBlob blockBlob = container.GetBlockBlobReference(postedFile.FileName);
    blockBlob.UploadFromStream(postedFile.InputStream);
}

我们已经尝试在上传之前强制设置ContentType属性，如下所示:

blockBlob.Properties.ContentType = "image/png";