How to add attachment to ir.attachment through json_rpc

Yes ced gave the solution but to make it work with flutter the trick is to give an object instead of a string :

var kk = {'__class__': 'bytes', 'base64': value};

instead of :

String kk = "{'__class__': 'bytes', 'base64':" + value+ "}";

With a string value I had the same error message :

Unicode-objects must be encoded before hashing