I just started the Quickstart with .NET v12 SDK https://docs.microsoft.com/da-dk/azure/storage/blobs/storage-quickstart-blobs-dotnet
But i cant seem to find out how
Please try this override of UploadAsync method. This method will upload and set the content type in a single network call.
Here would be your code:
var httpHeaders = new BlobHttpHeaders { ContentType = contentType }); await blobClient.UploadAsync(stream, httpHeaders);