In the course of
CAM-4653, we introduced an additional form part named "valueType" when setting a binary-valued variable (i.e. of types Bytes or File). This is used to distinguish the actual type instead of guessing the type based on request metadata. The UI must be adapted to set the correct value type when making such a request.
See the docs for the requests here: http://stage.docs.camunda.org/manual/develop/reference/rest/execution/local-variables/put-local-variable-binary/