I've started playing with Qlik Sense extensions these days and pretty much I'm happy with the process in overall.
You can develop extensions using Workbench (if you have Qlik Sense Desktop installed) or any text editor. But if you have Sense Server installed you need to develop directly on the server or upload your files every time a change is made.
From 7-8 months I'm a proud owner of Chromebook and I'm trying to use it as many as possible. At this point I can use it as my default laptop for work.
In terms of Sense extensions development I've used it to login to the server via RDP and edit the extensions there.
From few days I'm trying new approach:
I've installed FTP server on the server and made my home folder to be the Sense extensions folder. Then using ShiftEdit to edit the files and on each save ShiftEdit upload the edited files.
In my case I'm using ShiftEdit but other tools can be used. Almost all modern text editors have FTP sync modules. I've tested this approach with Notepad++, Atom.io, Sublime and Adobe Brackets and the result was the same.
If you don't want to use 3-rd party tools and you are ok with no using advanced text editor features you can use sFTP Client app (not free. £1.89 one time) for Chrome/Chrome OS.
Also found interesting tool that will watch it in the future - Zed
Hope you like it!