How to upload ZIP File from API Management to Blob Storage

1. I am sending a POST Request on one of my Azure Api Mangement APIs

2. In this post request there is a json body which contains base64 encoded data (zip file) like in this example:

   { "foo": "bar", "data": "your-base64-string" }

3. In the API Policy, I want to send a separate request to the Azure Storage REST API to save the above mentioned base64 string as a zip file.

Now the problem: I can successfully send a request to the Azure Storage REST API and download the resulting zip file via this statement:

<set-body>@{
var base64String = (string)context.Variables.GetValueOrDefault<JObject>("body")["data"]) ;
var bytes = Convert.FromBase64String(base64String); 
var ascii = Encoding.ASCII.GetString(bytes); 
return ascii; 
}</set-body> 

The thing is, I can download and open the zip file but I cannot extract the content of it as the archive got damaged.

Here is an example base64 encoded zip file string (ZIP File containing a text.txt):

UEsDBBQAAAAIAL1ZxE7MTbG/EwAAABMAAAAIAAAAdGVzdC50eHQLycgsVgCiRIWS1OKStMycVD0AUEsBAh8AFAAAAAgAvVnETsxNsb8TAAAAEwAAAAgAJAAAAAAAAAAgAAAAAAAAAHRlc3QudHh0CgAgAAAAAAABABgAI8Ac2LUa1QEjwBzYtRrVAZFuI861GtUBUEsFBgAAAAABAAEAWgAAADkAAAAAAA==

Here is my code for posting it to the Rest API:

<send-request mode="new" response-variable-name="putStorageRequest" timeout="5" ignore-error="true">
            <set-url>@((string)context.Variables["blobUrl"])</set-url>
            <set-method>PUT</set-method>
            <set-header name="x-ms-date" exists-action="override">
                <value>@((string) context.Variables["date"] )</value>
            </set-header>
            <set-header name="x-ms-version" exists-action="override">
                <value>@((string) context.Variables["version"] )</value>
            </set-header>
            <set-header name="x-ms-blob-type" exists-action="override">
                <value>BlockBlob</value>
            </set-header>
            <set-header name="Content-Type" exists-action="override">
                <value>application/zip</value>
            </set-header>
            <set-header name="Authorization" exists-action="override">
                <value>@{
                        string body = context.Request.Body.As<string>(preserveContent: true);
                        string contentType = "application/zip";
                        var base64String = (string)context.Variables.GetValueOrDefault<JObject>("body")["data"]) ;
            var bytes = Convert.FromBase64String(base64String); 
            var ascii = Encoding.ASCII.GetString(bytes)
                        var contentLength = ascii.Length;
                        var hmacSha256 = new System.Security.Cryptography.HMACSHA256 { Key = Convert.FromBase64String(context.Variables.GetValueOrDefault<string>("storageKey")) };
                        var payLoad = string.Format("{0}\n\n\n{1}\n\n{2}\n\n\n\n\n\n\nx-ms-blob-type:BlockBlob\nx-ms-date:{3}\nx-ms-version:{4}\n{5}", 
                            "PUT", 
                            contentLength,
                            contentType,
                            context.Variables["date"],
                            context.Variables["version"],
                            "/" + context.Variables.GetValueOrDefault<string>("storageAccountName") + context.Variables.GetValueOrDefault<string>("resource"));
                        return "SharedKey "+ context.Variables.GetValueOrDefault<string>("storageAccountName") + ":" + Convert.ToBase64String(hmacSha256.ComputeHash(Encoding.UTF8.GetBytes(payLoad)));
                    }</value>
            </set-header>
            <set-body>@{
                var base64String = (string)context.Variables.GetValueOrDefault<JObject>("body")["data"]) ;
                var bytes = Convert.FromBase64String(base64String); 
                var ascii = Encoding.ASCII.GetString(bytes); 
                return ascii;         
            }</set-body>
        </send-request>