Good suggestion, thanks!
So we've built a new beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
With this beta, if no file extension if given after the "@startuml filename", we add the corresponding one.
The only drawback to us is that it breaks somehow the compatibility. someone uses PlantUML today with "@startuml filename" without extension, he will have a slightly different result (because some extension will be added to the filename). However, since we do NOT append an extension if some is already present, we expected that this will impact very few people, because probably people are putting extension to filename.
Tell us if it's working for you!