File form redirection

[cs0511]

I have a form with file field and remote: true.

The request format of this form is ["application/javascript", "application/x-javascript"].

On backend, I check user's permission, and redirect to another url.

But, for somehow, the new request(the redirected url) format is changed to ["application/xhtml+xml"].

If I remove the file field of the form and submit, the new request format will be kept, which is still ["application/javascript", "application/x-javascript"].

Started POST "/s3_objects" for ::1 at 2021-03-19 07:50:44 +0800
Processing by S3ObjectsController#create as JS
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"WL+YNfGcSbunkL9L1n9sxiaj5wb6pn76TW0rr2yjg12OAI2Ukm7XMEAE7HcjRLdxEA9xppGrVqym6x8tziWlAg==", "s3_object"=>{"name"=>"Screen Shot 2021-03-16 at 7.59.20 AM.png", "description"=>"", "test_at"=>"", "folder_id"=>"142", "file"=>#<ActionDispatch::Http::UploadedFile:0x00007fe575392328 @tempfile=#<Tempfile:/var/folders/2c/_xjwwtjd0gj9ch2384jr77kw0000gn/T/RackMultipart20210319-8891-sfrrd3.png>, @original_filename="Screen Shot 2021-03-16 at 7.59.20 AM.png", @content_type="image/png", @headers="Content-Disposition: form-data; name=\"s3_object[file]\"; filename=\"Screen Shot 2021-03-16 at 7.59.20 AM.png\"\r\nContent-Type: image/png\r\n">}, "remotipart_submitted"=>"true", "X-Requested-With"=>"IFrame", "X-HTTP-Accept"=>"text/javascript, application/javascript, application/ecmascript, application/x-ecmascript, */*; q=0.01"}


Redirected to http://localhost:3000/subscriptions/trial_expired
Filter chain halted as :check_subscription! rendered or redirected
Completed 302 Found in 76ms (ActiveRecord: 55.8ms)


Started GET "/subscriptions/trial_expired" for ::1 at 2021-03-19 07:50:44 +0800
Processing by SubscriptionsController#trial_expired as HTML

I see this gem does a special process for file field, https://github.com/rails/jquery-ujs/blob/master/src/rails.js#L502,

Is it by design that the request format is changed?