CI3 file upload via ajax with csrf

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Are you adding a hidden field to #documents-form with the CSRF name/value pair?

You should be. If you do then FormData(this) will pick up all form's fields without you having to explicitly append them to the formData object.

Remember that the CSRF hash will change each time the server receives a POST.  So the value of the CSRF field on the form needs to be updated if you plan to make another POST without first making a GET to refresh the whole page.

Yes, I am doing that.