image_encoder.rb

# Create a helper lib
# e.g. lib/image_encoder.rb
module ImageEncoder
  def self.call(image_data, resource_name, model)
    image_data.match(/(image\/[a-z]{3,4});base64,(.*)/) do |match|
      content_type = match[1]
      decoded_data = Base64.decode64(match[2])

      temp = Tempfile.open("image", encoding: "ascii-8bit")
      temp.write(decoded_data)
      temp.rewind
      model.send(resource_name).attach(content_type: content_type, io: temp, filename: "img.png")
    end
  end
end

# Then call from a model file, such as: app/models/organization.rb
class Organization < ApplicationRecord
  include ImageEncoder
  has_one_attached :logo

  def logo_raw=(logo_image_data)
    ImageEncoder.call(logo_image_data, "logo", self)
  end

  def logo_url
    if self.logo.attachment
      self.logo.attachment.service_url
    else
      "https://some-default-path-to-image.png"
    end
  end
end