The DownloadFile function receives a JSON object in the body via an HTTP POST request, as follows:

{
    "url": "https://d365bcfilestorage.blob.core.windows.net/d365bcfiles/MasteringD365BC.png",
    "fileType": "image/png",
    "fileName": "MasteringD365BC.png"
}

This function retrieves the details of the file to download from the request body and calls the DownloadBlobAsync function. Then, it returns the content of the downloaded file (Base64-encoded string):

[FunctionName("DownloadFile")]
public static async Task<IActionResult> Download(
    [HttpTrigger(AuthorizationLevel.Function, "post", Route = null)] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");
    try
    {
        string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
        dynamic data = JsonConvert.DeserializeObject(requestBody);
        string url = data.url;
        string contentType = data.fileType;
        string fileName = data.fileName;
        byte[] x = await DownloadBlobAsync(url, fileName);
        //Returns the Base64 string of the retrieved file
        return (ActionResult)new OkObjectResult($"{Convert.ToBase64String(x)}");            
    }
    catch(Exception ex)
    {
        log.LogInformation("Bad input request: " + ex.Message);
        return new BadRequestObjectResult("Error on input parameter (object): " + 
            ex.Message);
    }
}

DownloadBlobAsync is the function that connects to the Azure Storage Blob container, checks for the file, and (if it is found) returns the byte array (stream) of this file:

public static async Task<byte[]> DownloadBlobAsync(string url, string fileName)
{
    CloudStorageAccount storageAccount =
        CloudStorageAccount.Parse(BLOBStorageConnectionString);
    CloudBlobClient client = storageAccount.CreateCloudBlobClient();
    CloudBlobContainer container = client.GetContainerReference("d365bcfiles"); 
    CloudBlockBlob blob = container.GetBlockBlobReference(fileName);
    await blob.FetchAttributesAsync();
    long fileByteLength = blob.Properties.Length;
    byte[] fileContent = new byte[fileByteLength];
    for (int i = 0; i < fileByteLength; i++)
    {
        fileContent[i] = 0x20;
    }
    await blob.DownloadToByteArrayAsync(fileContent, 0);
    return fileContent;
}