My fix is at https://github.com/openmrs/openmrs-module-webservices.rest/commit/c01a314b14751f71eb0495b2d36e241e0c8d1845
It works for all handlers, which accept and set InputStream in ComplexData.data (at least all in openmrs-api do) when requesting RAW_VIEW.
Please see 2 tests presenting possible POSTs. One is for embedding base64 encoded data in json and the other is for uploading a file. You always fetch a file by requesting /obs/{uuid}/value.