As it stands now, you cannot specify mime-type when posting base64 encoded complex obs.
We should support data URI scheme for complex obs
This would mean that Base64-encoded values included in the JSON body when posting a complex value could include the mime type. For example: "data:image/png;base64,<base64 data>"
When posting raw data using form-data, the mime-type should be passed (and received) in the Content-type header.
Fix needs to go to https://github.com/openmrs/openmrs-module-webservices.rest/blob/master/omod-1.8/src/main/java/org/openmrs/module/webservices/rest/web/v1_0/resource/openmrs1_8/ObsResource1_8.java#L318-L321