project.rb

ActiveAdmin.register Project do
    permit_params :name, :content, :price, :image
    
    show do |t|
        attributes_table do
            row :name
            row :content
            row :price
            row :image do
                project.image? ? image_tag(project.image, height: '100') : content_tag(:span, "No photo yet")
            end
        end
    end
    
    form :html => { :enctype => "multipart/form-data" } do |f|
        f.inputs do
            f.input :name
            f.input :content
            f.input :price
            f.input :image, as: :file, input_html: {multiple: true}
        end
        f.actions
    end
end