Extensions are very powerful and you can automate a lot of stuff but sometimes your extensions need to collect data and store it locally.
Here is a good article on how to save a file on disk.
There are ways to use the equivalent to save as or to issue a download which will prompt the user to download. However the alternative is to use an XMLHTTPRequest to grab the data.
You can see this in action in the Drag To Save Images
project on GitHub.
Are you are a member? You can get additional help by emailing me at firstname.lastname@example.org
Please note that for more advanced extensions that we may refer you to a paid mentor. For simpler extensions or popular issues we may be able to assist directly.