save base64 file as uploaded

base64_uploaded_file.rb

class XXXController < ApplicationController
  def create
    moment = Moment.new photo: preprocess_uploaded_photo
  
    # ...
  end
  
  def preprocess_uploaded_photo
    encoded_photo_data = params[:moment][:photo]
  
    image_data = Base64.decode64(encoded_photo_data['data:image/png;base64,'.length .. -1])
    tempfile = Tempfile.new ['upload', 'png']
    tempfile.binmode
    tempfile.write(image_data)
  
    ActionDispatch::Http::UploadedFile.new(tempfile: tempfile,
      filename: 'upload.png')
  end
end