We need to set it's property Content type through Blob Options class.
PHP :
namespace - use MicrosoftAzure\Storage\Blob\Models\CreateBlobOptions;
//use code where you are creating blob
$opts = new CreateBlobOptions();
//$opts->setCacheControl('test');
$opts->setContentEncoding('UTF-8');
$opts->setContentLanguage('en-us');
//$opts->setContentLength(512);
$opts->setContentMD5(null);
$opts->setContentType($mimeType);
$blobRestProxy->createBlockBlob($containerName, $indexFile, $content,$opts);
It will work in git package : "microsoft/windowsazure": "^0.5"
In C#
entryData.DestinationBlob.Properties.ContentType = "image/jpeg";
entryData.DestinationBlob.SetProperties();