I dont think its possible, a workaround would be to write the fme_basename into a file (csv) and import it via the attributefilter.
Hope this helps
Ie if you first run with the files myfile1.txt and myfile2.txt they are all connected OK:
However when you use it the next time with myfile3.txt you will anyway have to create a new line on the AttributeFilter and connect this to your desired output.
Have I understood correctly? Why do you want to use an AttributeFilter on fme_basename? if this is for writing to different output-files you can use fanout on fme_basename instead:
I have used the csv option in the past, I was just wondering if there was an option I'd missed somewhere.