GreenLight

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This extension prevents you from uploading NSFW pictures.This extension prevents you from uploading NSFW pictures. An alert will pop-up if you try to upload any pictures with NSFW content to any website, and that upload will be prevented.

To scan images, this extension uses an API service provided by Clarifai.
Make an account at https://developer.clarifai.com/signup/ and register an app that uses the NSFW model.
Use the resulting client id and client secret in the popup that appears when you click the GreenLight button on the right side of the Omnibar.