There is only one file type that uses the .NEX extension:
A NEX file is a .ZIP file saved in the open Navigator Extension (NEX) format for browser add-on development. It contains all the files and folders for an extension and is used to install browser add-ons, like adblockers. NEX files support a large amount of the Google Chrome web browser (Chromium) extensions.
If you have created the .OEX file format for Opera 11 and 12 web browsers you should convert them to the Chromium-based Opera 15 browser format, which is NEX and .CRX.
NOTE: Opera runs Chromium's CRX files as long as the extension uses the chrome specific APIs that Opera supports.
Updated: March 31, 2017