@wyclif, have a look at 2 tests at https://github.com/openmrs/openmrs-module-webservices.rest/blob/master/omod-1.9/src/test/java/org/openmrs/module/webservices/rest/web/v1_0/controller/openmrs1_9/ObsController1_9Test.java#L119-L162
It should be then clear that you can post your file either as base64 embedded in json or “as is” using the multipart/form-data approach. You always fetch the file as is from /obs/(uuid)/value (it’s never encoded in base64 when you fetch it back).