Paperclip + rails 4.2 - image_file_name is null

I've setup paperclip (v 4.2.1) + aws-sdk-v1 gem, on a Rails 4.2.1 app. The model is setup thusly:

Model:

class Photo < ActiveRecord::Base

  attr_accessor :image_file_name
  has_attached_file :image, :styles => { :medium => "300x300>", :thumb => "100x100>" }, :default_url => "/images/:style/missing.png"
  validates_attachment_content_type :image, :content_type => ["image/jpg", "image/jpeg", "image/png", "image/gif"]

end

Form:

 = form_for @photo_set_new_photo, :html => { :multipart => true } do |f|
    = f.file_field(:image, {class: 'photo-upload', accept:"image/*;capture=camera"})

Controller:

def create
  @photo = Photo.new(photo_params)

  respond_to do |format|
    if @photo.save
      format.html { redirect_to @photo, notice: 'Photo was successfully created.' }
      format.json { render :show, status: :created, location: @photo }
    else
      format.html { render :new }
      format.json { render json: @photo.errors, status: :unprocessable_entity }
    end
  end
end

def photo_params
  params.require(:photo).permit(:category_id, :photo_set_id, :image)
end

So pretty much standard cookie cutter what you would expect. When this form is submitted, I have this in the logs:

Started POST "/photos" for 127.0.0.1 at 2015-05-04 11:47:04 +0200
Processing by PhotosController#create as JSON
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"...", "photo"=>{"category_id"=>"", "image"=>#<ActionDispatch::Http::UploadedFile:0x007ffe5c421ed8 @tempfile=#<Tempfile:/tmp/RackMultipart20150504-20205-c0g1d6.png>, @original_filename="antaeater-birthday.png", @content_type="image/png", @headers="Content-Disposition: form-data; name=\"photo[image]\"; filename=\"antaeater-birthday.png\"\r\nContent-Type: image/png\r\n">}}
Command :: file -b --mime '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-1li6xz7.png'
Command :: identify -format '%wx%h,%[exif:orientation]' '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]' 2>/dev/null
Command :: identify -format %m '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]'
Command :: convert '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]' -auto-orient -resize "300x300>" '/tmp/6025e18d9f999e893c1840772ad7c87b20150504-20205-v780mg'
Command :: identify -format '%wx%h,%[exif:orientation]' '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]' 2>/dev/null
Command :: identify -format %m '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]'
Command :: convert '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-2uc2d6.png[0]' -auto-orient -resize "100x100>" '/tmp/6025e18d9f999e893c1840772ad7c87b20150504-20205-1qlvsdh'
   (0.2ms)  BEGIN
Command :: file -b --mime '/tmp/3bdad8b82d38ab9d77cab8cd85c2877120150504-20205-3p5x8o.png'
  SQL (0.3ms)  INSERT INTO "photos" ("image_content_type", "image_file_size", "image_updated_at", "image_file_name", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"  [["image_content_type", "image/png"], ["image_file_size", 1241569], ["image_updated_at", "2015-05-04 09:47:04.866533"], ["image_file_name", nil], ["created_at", "2015-05-04 09:47:05.173238"], ["updated_at", "2015-05-04 09:47:05.173238"]]
[paperclip] saving /photos/images/000/000/008/original/antaeater-birthday.png
[AWS S3 200 2.681968 0 retries] put_object(:acl=>:public_read,:bucket_name=>"bucket-development",:content_length=>1241569,:content_type=>"image/png",:data=>Paperclip::UploadedFileAdapter: antaeater-birthday.png,:key=>"photos/images/000/000/008/original/antaeater-birthday.png")  

[paperclip] saving /photos/images/000/000/008/medium/antaeater-birthday.png
[AWS S3 200 0.17464 0 retries] put_object(:acl=>:public_read,:bucket_name=>"bucket-development",:content_length=>145007,:content_type=>"image/png",:data=>Paperclip::FileAdapter: 6025e18d9f999e893c1840772ad7c87b20150504-20205-v780mg,:key=>"photos/images/000/000/008/medium/antaeater-birthday.png")  

[paperclip] saving /photos/images/000/000/008/thumb/antaeater-birthday.png
[AWS S3 200 0.074367 0 retries] put_object(:acl=>:public_read,:bucket_name=>"bucket-development",:content_length=>18256,:content_type=>"image/png",:data=>Paperclip::FileAdapter: 6025e18d9f999e893c1840772ad7c87b20150504-20205-1qlvsdh,:key=>"photos/images/000/000/008/thumb/antaeater-birthday.png")  

The file is saved to the S3 bucket (I can see it on the management console) but the foto record in the database looks like this:

 id | image_file_name | image_content_type | image_file_size  | image_updated_at
 8  |       NULL      |   image/png        | 1241569      | 2015-05-04 09:47:04.866533

As you can see - the image_file_name is null, which is already what the logs are telling us. The question is - why?