Please select a file to upload using StorageApi.Upload operation
This is an example of discouraged usage of GroupDocs Javascript SDK.
A few reasons why it's not recommended way:
Javascipt code is inlined in an html - there is no excuse for that.
Example uses groupdocs-javascript.js which is a concatenated version of all JS files that comprise the SDK. So if you are using only StorageApi you are still loading whole SDK into the browser.
When using groupdocs-javascript.js all objects are added to groupdocs namespace in global scope (window), thus polluting global scope, which is a bad practice.