I will love forever anyone who can help me towards the solution to this problem: https://stackoverflow.com/questions/55422101/how-to-get-a-netsuite-file-as-a-byte-array-in-a-suitescript-2-suitelet

I don't think ArrayBuffer is part of ES5 so no 'native' byte array. You do get Base64 encoded file contents so maybe you can send that with 'Content-Tranfer-Encoding'?