NiFi Expression Language in UpdateAttribute

[Russell Bateman]

I wnat to create an attribute, format-type, which I hoped would be set 
based on the in-coming flowfile name:

    ${ ${filename:endsWith( 'xml' )} : ifElse( 'XML', 'JSON' ) }
If the in-coming flowfile name is /sample.xml/, the result is 
nevertheless and always

format-type = JSON

I want it to be XML, obviously. What have I screwed up in my NEL syntax?

(Yes, I have tried taking out all the white space.)

Thanks,
Russ