Which browser plugin technology to use for cross platform design?

I mean ActiveX is IE only and dead, and so java applets are dead. Flash is still not present in mobile browsers, Mo开发者_高级运维zilla is present only on 1 phone, so XPCOM is also not an option.

How do you write a plugin that need access to file system, and do some other things, like unzip files, upload, download, etc.

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Forget about client side plugins. Most of that can be handled on the server side. Then for anything that really does need a client side implementation you can use Javascript.