Fineuploader is an excellent frontend javascript library for supporting full-featured uploading capabilities such as concurrent file chunking and file resume. Our use case involves uploading extremely large files such as genomic DNA sequencing data including FASTQs, BAMs and VCFs. But the javascript library does run on it's own out of the box. You will still need to implement some server-side code to handle the file uploading.
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:
https://github.com/FineUploader/server-examples
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: