filter:register.check should do what you want, it gets fired when the user tries to register.
Using the option "Allow users to upload regular files" question
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How would I allow only specified extensions? I don't want users uploading just any ol file, it could be a virus being uploaded, who knows...
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added an issue for this
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Thanks! It is the right step toward allowing for images being saved on the server vs imgur. That's why I am using it among other reasons as well.