How copy selected property (not all) from camel to activiti - variables

Question

Camel scenario:
read file
do something e.g store content in database
run activiti process and pass variable from previous step
RouteBuilder:
from("file:/home/work/Inbox")
.to("bean:sourceFileService?method=storeFile")
.to("activiti:Receive?copyVariablesFromProperties=true")
During call bean sourceFileService property 'sourceFileId' is set.
This variable should be transferred to the process Receive as variable.
If I don't use copyVariablesFromProperties any variable is not set in process.
One the other side when copyVariablesFromProperties=true then camel try pass all properties and exception occurs:
ActivitiException: couldn't find a variable type that is able to serialize GenericFile
(because one of property is object represents read file, there are 7 other unwanted property )
How pass only selected property to activiti endpoint or in camel any next 'to' ?

Answer 1

change sourceFileService's storeFile method signature as follows;
public String storeFile(... your other params, #OutHeaders Map headers) {
...
headers.put("sourceFileId", "32132132");
....
}
and you can access the set sourceFileId in your activiti endpoint

Answer 2

I have found that the use of copyVariablesFromProperties is not necessary.
The same is achieved by
.setBody().properties()
.to("activiti:Receive")
When in body is Map camel set variables for activiti process using that map.
But I still get exception for the same reason (pass unwanted, not serializable object).
The only solution I have found is
from("file:/home/work/Inbox")
.to("bean:sourceFileService?method=storeFile")
.setBody(method(Helper.class))
.to("activiti:Receive")
where
public class Helper {
#Handler
public Map getProcessVariables(Exchange exchange) {
Map<String, Object> variables = new HashMap<String, Object>();
variables.put("sourceFileId, exchange.getProperty("sourceFileId"));
return variables;
}
}