How to correlate on part of filename?

The Scenario i’m facing:

Send a file G0001.TXT (using Passingthrough) and then get a response file either to be G0001.ACK or G0001.ERROR.

The orchestration i set up is like this:

START
|
Receive Gfile (activate)
|
Send Gfile (initialize correlation)
|
Listen (for response)
_________|____________
| |
Receive ACK Receive ERROR
(Follow Correlation) (Follow correlation)
| |
Send ACK Send ERROR
|______________________|
|
END

How can i set up the correlation properly? Since file extension is different.

I tried just use the correlation to be FILE.ReceivedFileName, Files can be transmitted but got warnings like this:

\”The FILE send adapter cannot open file C:\\TestLocation\\G0005.INT for writing.\”