I want to pass attatchment with sms using rest api in apex

I have written apex code to send sms its working fine but now i need to send the attatchment with same

I have converted same in Blob But not getting how to pass same as parameter in smsbody

String smsBody = '{"from":{"phoneNumber":"' + fromPhoneNumber + '"},"text":"' + message+ '","to":[{"phoneNumber":"'+ tonumber+'"}]}';