Using new sdk, one can set the content type as follows(i have set the content type for gif image in this example).
$blobRestProxy = ServicesBuilder::getInstance()->createBlobService($connectionString);
//upload
$blob_name = "image.gif";
$content = fopen("image.gif", "r");
$options = new CreateBlobOptions();
$options->setBlobContentType("image/gif");
try {
//Upload blob
$blobRestProxy->createBlockBlob("containername", $blob_name, $content, $options);
echo "success";
} catch(ServiceException $e){
$code = $e->getCode();
$error_message = $e->getMessage();
echo $code.": ".$error_message."<br />";
}