Why the gzipped file uploaded to the azure blob container with a custom file extension is being downloaded as .gz via the generated sas?

Here is my method on generating the SAS:

        private string GenerateSasBlob(BlobClient blobClient)
        {
            BlobSasBuilder sasBuilder = new BlobSasBuilder()
            {
                BlobContainerName = blobClient.GetParentBlobContainerClient().Name,
                BlobName = blobClient.Name,
                Resource = "b",
                StartsOn = DateTimeOffset.UtcNow,
                ExpiresOn = DateTimeOffset.UtcNow.AddMonths(1),
                Protocol = SasProtocol.Https,
                ContentType = "mygzip"
            };
            sasBuilder.SetPermissions(BlobSasPermissions.Read);

            return blobClient.GenerateSasUri(sasBuilder).ToString();
        }

I thought that by specifying the ContentType fixes it however the generated sas still downloads as .gz instead of the expected .mygzip.

Though looking into side-by-side the contents inside of the compressed file is the same but what I need is it should be .mygzip when downloaded. Any idea on how?

Considering you would want the blob to be downloaded with a different extension (mygzip instead of gz), essentially you would want the blob to be downloaded with a different name.

In that case, the response header you would want to overwrite is Content-Disposition instead of Content-Type.

Your code would be something like:

private string GenerateSasBlob(BlobClient blobClient)
{
    var newBlobName = blobClient.Name.Replace(".gz", ".mygzip");//Change the extension here in the name
    BlobSasBuilder sasBuilder = new BlobSasBuilder()
    {
        BlobContainerName = blobClient.GetParentBlobContainerClient().Name,
        BlobName = blobClient.Name,
        Resource = "b",
        StartsOn = DateTimeOffset.UtcNow,
        ExpiresOn = DateTimeOffset.UtcNow.AddMonths(1),
        Protocol = SasProtocol.Https,
        ContentDisposition = $"attachment; filename=\"{newBlobName}\""
    };
    sasBuilder.SetPermissions(BlobSasPermissions.Read);

    return blobClient.GenerateSasUri(sasBuilder).ToString();
}

Now when a user clicks on the SAS URL, the blob will be downloaded with "mygzip" extension.

More information on Content-Disposition header can be found here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition.