When setting a typed Variable (e.g. a file like in the code example below) this information is lost when correlating a Message:
BpmPlatform.getDefaultProcessEngine().getRuntimeService().createMessageCorrelation("MSG_DOCUMENT_RECEIVED")
...
.setVariables(variables)
.correlate();

Probably because this is a Map instead of a VariableMap:
https://github.com/camunda/camunda-bpm-platform/blob/master/engine/src/main/java/org/camunda/bpm/engine/impl/MessageCorrelationBuilderImpl.java#L39

My variable:
VariableMap variables = Variables.createVariables().putValueTyped(
"document",
Variables
.fileValue((String)documentVariable.getValueInfo().get("filename"))
// Base64.decodeBase64(
.file(((String)documentVariable.getValue()).getBytes("UTF-8"))
.mimeType((String)documentVariable.getValueInfo().get("mimeType"))
.create());