UploadsController: NoMethodError: undefined method `filename' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/648515/

NoMethodError: undefined method `filename' for nil:NilClass
  file_uploader.rb:198:in `secure_url'
    File.join('/uploads', @secret, file.filename)
  file_uploader.rb:128:in `to_h'
    url:      secure_url,
  uploads_actions.rb:15:in `block (2 levels) in create'
    render json: { link: uploader.to_h }
  action_controller/metal/mime_responds.rb:201:in `respond_to'
    response.call if response
  uploads_actions.rb:12:in `create'
    respond_to do |format|
...
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NoMethodError: undefined method `filename' for nil:NilClass