The expression for the idempotent consumer can be anything that Camel supports such as any of these languages: http://camel.apache.org/languages
So what you want is the message body as the correlation expression, and for that you can use
<simple>${body}</simple>
But as the file component routes the file as a java.io.File handle, you would need to load the content into memory for comparision, so you can do
<from uri="file://inputFolder" />
<convertBodyTo type="String" />
<idempotentConsumer messageIdRepositoryRef="fileStore">
<simple>${body}</simple>
...