Hi seanmerron,
this seems the be a typical issue with the REST APIs, mssing documentation on what Content-Type should be used,
although I've seen this for BSM only - until now.
You didn't mention what exact call you used and failed on, so I did a more global search and found these types:
MediaType.MULTIPART_FORM_DATA
MediaType.APPLICATION_JSON
So simply give it a try with
Content-Type: multipart/form-data
or
Content-Type:application/json
one of it should do the trick.
Greetings
Siggi