As the question describe. I have exported geojson to multiple files but I realize they are all missing the extension which make the program I will use the file in unable to read them. Is this a bug or how do I fix it?
Check the actual contents of the attribute areaname just before it arrives at the writer, e.g. using a breakpoint, Logger or Inspector.
Did that with the logger. The attribute name is as "expected" without the (.geojson) in the name. A bit surprised how it evaluate the name. Maybe I am still doing something wrong. But I have just added @Value(areaname).geosjon in the expression option