The URL used for the generated request was
`/ckeditor/pictures&responseType=json`. This is a known issue in the
ckeditor gem, and it's suggested to add a `?` at the end of the URL in
order to fix it.
I haven't added a test for this case since simulating dropping a file in
the browser with Selenium/Capybara seems to be quite tricky and I
haven't found a solution guaranteed to correctly emulate what users do.
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