there are so many great extension on qlik branch but here is my problem: We are a company that develops application for different customers. How can I be sure that my extension will be maintained. Who am I supposed to get in touch with if my extension suddenly stops working (after an update for instance)?
Extensions available on Qlik Branch are not maintained by Qlik - they are maintained by the extension developer only. If a developer decides to stop updating the extension or doesn't provide support, you will have to update it yourself - or find another extension to swap it out for.
There is a trusted extension developer programme (Trusted Extension Developer Program) that's just started, and also a number of paid-for extensions which offer support (for a fee - for example, the extension library built by VizLib).