Azure Function blob input binding with a servicebus trigger

Looks like this isn't supported for Service Bus Trigger the way it is for Queue Trigger.
You could raise a feature request on UserVoice for this.

But one thing to note is that this limitation holds only for non-JSON messages. If you send a JSON message instead, it will get parsed as documented.

Your function could be something like this

using System;
using System.IO;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;

namespace funk_csharp_queue
{
  public class QueueMsg
  {
    public string filename { get; set; }
  }
  public static class ServiceBusTrigger
  {
    [FunctionName("ServiceBusTrigger")]
    public static void Run(
      [ServiceBusTrigger("myqueue", Connection = "ServiceBusConnection")] QueueMsg myQueueItem,
      [Blob("samples-workitems/{filename}", FileAccess.Read)] String myBlob,
      ILogger log)
    {
      log.LogInformation($"C# Service Bus trigger function processed: {JsonConvert.SerializeObject(myQueueItem)}");
      log.LogInformation($"C# Blob input read: {myBlob}");
    }
  }
}

And the message you send in the Service Bus Queue / Topic would be something like this

{
  "filename": "11c8f49d-cddf-4b82-a980-e16e8a8e42f8.json"
}

Make sure you set the Content Type to application/json.

I don't think that's directly possible as you would like to. I would just use a CloudBlobContainer input binding to work around this.

[Blob("email-messages", FileAccess.Read, Connection = "BlobConnectionString")] CloudBlobContainer blobContainer

Then use the blobContainer to read the blob in your function using the inputMessage.

CloudBlob mailBlob = blobContainer.GetBlobReference(inputMessage);