There's already a Github repository with many examples of server-side implementations for the popular programming languages like Java, Python, PHP, node.js etc... which can be found here:
However, I could not find any examples for a Grails implementation, so once again, I rolled up my own solution which can be found below. For this implementation I've only focused on file concurrent chunking and file resume. Other features such as file deletion we're omitted on purpose, but that's not to say you couldn't modify it to support the remaining features of fineuploader.
And for the GSP I have something like this: