Please support .pu as an extension in the GUI next to txt, tex, etc.
Also "claim"/register this extension which is still fee at:
http://file-extension.net/seeker/
http://filext.com/submitext.php
https://en.wikipedia.org/wiki/Alphabetical_list_of_file_formats_%28M%E2%80%93R%29
http://www.fileinfo.com/contact?form=add
http://dotwhat.net/site/add
http://www.cryer.co.uk/file-types/index.htm
http://mark0.net/contacts.html
http://forum.file-extensions.org/new-file-extensions-and-suggestions/
And update documentation of http://plantuml.sourceforge.net/command_line.html
If you want, you can contact me to help out with the registration of .pu at all these site.
Implementing this can allow for quick automated rendering, previewing and even visual "diff"-ing in GitHub based on the file extension without having to inspect files first. The way PlantUML is finding files to process is fine, but an extension for filtering out PlantUML files without inspecting them internally is a must have for integration with e.g. GitHub on file name bases.