Spring Integration - Spel Mapping of Gateway Parameters - spring-integration

Question

I have the following interface, who's implementation will encrypt a file at a specific path.
package xx.messaging.fileTransfer;
import org.springframework.integration.annotation.Gateway;
import org.springframework.integration.annotation.Header;
import org.springframework.integration.annotation.Payload;
/**
* <H1>FileEncryptionService</H1>
*/
public interface FileEncryptionService {
/**
* Generates a new encrypted filename based on the input, calls the default method and returns
* the encrypted file name
* #param srcFilename
* #return
* #throws Exception
*/
#Gateway
public String encryptFile(#Payload String srcFilename) throws Exception;
/**
* Encrypt the a file
* #param srcFilename The source file name
* #param destFilename The target file name
*/
#Gateway
public void encryptFile(#Payload String srcFilename, #Header("encryptedFilename") String destFilename);
}
The service will be called via the spring integration and is registered in the context as a gateway via
<int:gateway service-interface="lu.scoteqint.messaging.fileTransfer.FileEncryptionService"/>
The implementing bean and service-activator are registered as
<beans:bean id="fileEncryptionService" class="xx.messaging.fileTransfer.impl.CommandLineEncryptionService"/>
<int:service-activator
input-channel="file1"
output-channel="file2"
expression="#fileEncryptionService.encryptFile(payload)"/>
I'm expecting that the message payload is the string path to the file, since the wire-tag log shows
2013-02-05 15:50:26,911 DEBUG [org.springframework.integration.handler.LoggingHandler] (task-scheduler-1) [Payload=.\src\test\resources\test\xx.xml][Headers={timestamp=1360079426907, id=1c3be020-fc6d-42ba-b3f0-5b963f76fb76}]
But it seems the 'service-activator' expression finds a File.
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1004E:(pos 31): Method call: Method encryptFile(java.io.File) cannot be found on lu.scoteqint.messaging.fileTransfer.impl.CommandLineEncryptionService type
at org.springframework.expression.spel.ast.MethodReference.findAccessorForMethod(MethodReference.java:182)
at org.springframework.expression.spel.ast.MethodReference.getValueInternal(MethodReference.java:106)
at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:57)
at org.springframework.expression.spel.ast.SpelNodeImpl.getTypedValue(SpelNodeImpl.java:102)
at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:102)
at org.springframework.integration.util.AbstractExpressionEvaluator.evaluateExpression(AbstractExpressionEvaluator.java:126)
at org.springframework.integration.util.AbstractExpressionEvaluator.evaluateExpression(AbstractExpressionEvaluator.java:86)
... 30 more
EDIT
The log detail
2013-02-05 16:26:42,737 DEBUG [org.springframework.integration.channel.DirectChannel] (task-scheduler-1) preSend on channel 'file1', message: [Payload=.\src\test\resources\test.xml][Headers={timestamp=1360081602736, id=877407f6-c5a2-4bea-9ec7-f970b09f08a8}]
Is there a way to ensure the parameter is mapped as a String rather than a File?