Fix file caching issue when form is resubmitted without file

Author: Skumring

lib/active_admin_import/model.rb

@@ -48,6 +48,7 @@ def initialize(args = {})
     end
 
     def assign_attributes(args = {}, new_record = false)
+      args[:file] = nil unless args.key?(:file)
       @attributes.merge!(args)
       @new_record = new_record
       args.keys.each do |key|

spec/import_spec.rb

@@ -588,4 +588,26 @@ def upload_file!(name, ext = 'csv')
       expect { add_author_resource(options) }.to raise_error(ArgumentError)
     end
   end
+
+  context 'when submitting empty form after validation error' do
+    let(:options) { {} }
+
+    before do
+      add_author_resource(options)
+      visit '/admin/authors/import'
+    end
+
+    it 'should NOT reuse cached file from previous submission' do
+      expect do
+        upload_file!(:author_broken_header)
+        expect(page).to have_content("can't write unknown attribute")
+      end.not_to change { Author.count }
+
+      # Second submission without selecting a file
+      expect do
+        find_button('Import').click
+        expect(page).to have_content(I18n.t('active_admin_import.no_file_error'))
+      end.not_to change { Author.count }
+    end
+  end
 end