How to post binary data

[JoshRagem]

I am trying to test an endpoint that accept a POST body of binary data.
I am generating the binary data with a little erlang module:

      <request subst="true">
        <http url="/endpoint"
              method="POST"
              version="1.1"
              contents="%%binary_gen:generate%%">
          <http_header name="authorization" value="bearer %%_token%%"/>
        </http>
      </request>

ebin/binary_gen.beam compiled from:

-module(binary_gen).
-export([generate/1]).

generate({_Pid, _DynData}) ->
    <<"mybinarydata">>.

but this request doesn't seem to be run. The three requests in the session before this request do run, however.