Getting file type in Blob Storage bound Azure Function

Question

I'm creating an Azure Function that is bound to my Azure Blob Storage container and triggers whenever a file is uploaded.

This particular container could have any type of file/blob in it e.g. image, PDF, Excel, MP4, etc.

I want to create different handlers that will do something depending on uploaded file type. For example, if it's an image file, I want to get its dimensions. If it's an MP4, I want to call another service to process the video, etc.

How do I get the file type from Stream? It's important to note that I cannot rely on file extension as in some cases, the extension may not be there. Is there a way for me to get the mimetype?

This is the standard BlogTrigger Azure Function code:

[FunctionName("FileManager")]
public static void Run([BlobTrigger("my-container/{name}", Connection = "myConnectionString")]Stream myBlob, string name, ILogger log)
{
    log.LogInformation($"C# Blob trigger function Processed blob\n Name:{name} \n Size: {myBlob.Length} Bytes");
}

I think you need to use CloudBlockBlob to receive the blob, and then you can get the mimetype of your blob. For the stream of the blob, you can download by DownloadToStreamAsync method. Please have a look of my answer. — Cindy Pau, Oct 21 '20 at 06:49

score 1 · Answer 1 · answered Oct 20 '20 at 23:56

You can get these properties.

The Content-Type (if set when uploaded) is returned in metaData argument.

[FunctionName("SomeFunction")]
public async Task RunFunctionFaceDetection(
    [BlobTrigger("%PhotosContainerName%/{blobname}.{blobextension}", Connection = "storage-conn")]Stream inputBlob,
    string blobName, //blob name
    string blobExtension, //blob extension - file extension
    string blobTrigger, // full path to triggering blob
    Uri uri, // blob primary location
    IDictionary<string, string> metaData) // user-defined blob metadata
{
    _logger.LogInformation($"RunFunctionAWSFaceDetection just started...");
    _logger.LogInformation($@"
            blobName      {blobName}
            blobExtension {blobExtension}
            blobTrigger   {blobTrigger}
            uri           {uri}
            metaData      {metaData.Count}");

    _logger.LogInformation($"C# Blob trigger function processed blob\n Name:{blobName} \n Size: {inputBlob.Length} Bytes");
}

You save my day! Thanks! – Geancarlo Murillo Nov 10 '21 at 01:51 — Geancarlo Murillo, Nov 10 '21 at 01:51

Cindy Pau · Answer 2 · 2020-10-21T06:45:20.977

No, please use CloudBlockBlob to receive the blob:

using System;
using System.IO;
using System.Text;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Extensions.Logging;
using Microsoft.WindowsAzure.Storage.Blob;

namespace FunctionApp24
{
    public static class Function1
    {
        [FunctionName("Function1")]
        public static void Run([BlobTrigger("test/{name}", Connection = "str")] CloudBlockBlob myBlob, string name, ILogger log)
        {
            string a = myBlob.Properties.ContentType;
            log.LogInformation(a);
        }
    }
}

And you can use below code to convert the blob to stream:

Stream stream = new MemoryStream();
myBlob.DownloadToStreamAsync(stream).Wait();

Getting file type in Blob Storage bound Azure Function

2 Answers2