Fix bugs with caching and format validation when importing files

lib/active_admin_import/model.rb

@@ -37,7 +37,7 @@ module CONST
     validate :file_contents_present, if: ->(me) { me.file.present? }
 
     before_validation :unzip_file, if: ->(me) { me.archive? && me.allow_archive? }
-    before_validation :encode_file, if: ->(me) { me.force_encoding? && me.file.present? }
+    after_validation :encode_file, if: ->(me) { me.force_encoding? && me.file.present? }
 
     attr_reader :attributes
 
@@ -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/fixtures/files/author_invalid_format.txt

@@ -0,0 +1,2 @@
+Name,Last name,Birthday
+John,Doe,1986-05-01

spec/import_spec.rb

@@ -588,4 +588,42 @@ 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
+
+  context 'when importing file with invalid format and auto force_encoding' do
+    let(:options) { { template_object: ActiveAdminImport::Model.new(force_encoding: :auto) } }
+
+    before do
+      add_author_resource(options)
+      visit '/admin/authors/import'
+    end
+
+    it 'should reject invalid file format before encoding' do
+      expect do
+        upload_file!(:author_invalid_format, 'txt')
+        expect(page).to have_content I18n.t('active_admin_import.file_format_error')
+      end.not_to change { Author.count }
+    end
+  end
 end