Remove unused code in Dragonfly field

lib/rails_admin/config/fields/types/dragonfly.rb

@@ -12,7 +12,6 @@ class Dragonfly < RailsAdmin::Config::Fields::Types::FileUpload
           RailsAdmin::Config::Fields::Types.register(self)
 
           register_instance_option :image? do
-            false unless value
             if abstract_model.model.new.respond_to?("#{name}_name")
               mime_type = Mime::Type.lookup_by_extension(bindings[:object].send("#{name}_name").to_s.split('.').last)
               mime_type.to_s.match?(/^image/)

spec/fixtures/test.txt

@@ -0,0 +1 @@
+test

spec/rails_admin/config/fields/types/drangonfly_spec.rb

@@ -5,6 +5,29 @@
 RSpec.describe RailsAdmin::Config::Fields::Types::Dragonfly do
   it_behaves_like 'a generic field type', :string_field, :dragonfly
 
+  let(:field) do
+    RailsAdmin.config('FieldTest').fields.detect do |f|
+      f.name == :dragonfly_asset
+    end.with(object: record)
+  end
+
+  describe '#image?' do
+    let(:file) { File.open(file_path('test.jpg')) }
+    let(:record) { FactoryBot.create :field_test, dragonfly_asset: file }
+
+    it 'returns true' do
+      expect(field.image?).to be true
+    end
+
+    context 'with non-image' do
+      let(:file) { File.open(file_path('test.txt')) }
+
+      it 'returns false' do
+        expect(field.image?).to be false
+      end
+    end
+  end
+
   describe 'with a model which does not extend Dragonfly::Model' do
     before do
       class NonDragonflyTest < Tableless