Google Chrome Extensions and NPAPI
If you want multi platform in one CRX, you should put them in this order within the manifest:
"plugins": [ { "path": "plugin-windows.dll" }, { "path": "plugin-linux.so" }, { "path": "plugin-mac.plugin" }]Remember uploading your NPAPI extension to the store will only work in Google Chrome Extensions. In the case of Google Chrome OS (which uses Google Chrome), you will not be able to use NPAPI.
But you should look into PPAPI which will be the recommended way doing plugin development for extension:
http://src.chromium.org/viewvc/chrome/trunk/src/ppapi/
In the meantime NPAPI in extensions, you need to follow this guide (which works great):
http://code.google.com/chrome/extensions/npapi.html
Just remember, do you really need to use NPAPI in your extension? If you could find a HTML5 workaround, it would be better.