CarrierWave::FormNotMultipart error when creating records using seed.rb file

Presuming that your Gem model has the image field as a Carrierwave uploader (that is, you've got mount_uploader :image, ImageUploader or similar in your Gem model), you can assign a ruby File object to your image attribute, not a string. Something like this:

gem = Demogem.new(data)
image_src = File.join(Rails.root, "/public/images/test2.png")
src_file = File.new(image_src)
gem.image = src_file
gem.save

---

In your overall code, you could either change your seed data (so your image property in your hash was set to File.new(File.join(Rails.root, "/public/images/test1.jpg")) or change the way you construct your new records so it doesn't use the image by default:

gems.each do |user, data|
    gem = DemoGem.new(data.reject { |k, v| k == "image" })
    gem.image = new File(File.join(Rails.root, data["image"]))

    unless DemoGem.where(name: gem.name).exists?
        gem.save!
    end
end

The CarrierWave wiki has some documentation on this, but it's not very extensive.