Resolve "Unable to import repository: undefined method `import_file' for nil:NilClass"

app/controllers/import/gitlab_projects_controller.rb

@@ -11,7 +11,7 @@ class Import::GitlabProjectsController < Import::BaseController
 
   def create
     unless file_is_valid?
-      return redirect_back_or_default(options: { alert: "You need to upload a GitLab project export archive." })
+      return redirect_back_or_default(options: { alert: "You need to upload a GitLab project export archive (ending in .gz)." })
     end
 
     @project = ::Projects::GitlabProjectsImportService.new(current_user, project_params).execute
@@ -29,7 +29,11 @@ class Import::GitlabProjectsController < Import::BaseController
   private
 
   def file_is_valid?
-    project_params[:file] && project_params[:file].respond_to?(:read)
+    return false unless project_params[:file] && project_params[:file].respond_to?(:read)
+
+    filename = project_params[:file].original_filename
+
+    ImportExportUploader::EXTENSION_WHITELIST.include?(File.extname(filename).delete('.'))
   end
 
   def verify_gitlab_project_import_enabled

app/controllers/projects_controller.rb

@@ -196,7 +196,7 @@ class ProjectsController < Projects::ApplicationController
 
   def download_export
     if @project.export_file_exists?
-      send_upload(@project.export_file)
+      send_upload(@project.export_file, attachment: @project.export_file.filename)
     else
       redirect_to(
         edit_project_path(@project, anchor: 'js-export-project'),

changelogs/unreleased/50944-unable-to-import-repository-undefined-method-import_file-for-nil-nilclass.yml

+---
+title: Fix import error when archive does not have the correct extension
+merge_request: 21765
+author:
+type: fixed

spec/controllers/import/gitlab_projects_controller_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 describe Import::GitlabProjectsController do
   set(:namespace) { create(:namespace) }
   set(:user) { namespace.owner }
-  let(:file) { fixture_file_upload('spec/fixtures/doc_sample.txt', 'text/plain') }
+  let(:file) { fixture_file_upload('spec/fixtures/project_export.tar.gz', 'text/plain') }
 
   before do
     sign_in(user)

spec/features/projects/import_export/export_file_spec.rb

@@ -49,6 +49,8 @@ describe 'Import/Export - project export integration test', :js do
 
       expect(file_permissions(project.export_path)).to eq(0700)
 
+      expect(project.export_file.path).to include('tar.gz')
+
       in_directory_with_expanded_export(project) do |exit_status, tmpdir|
         expect(exit_status).to eq(0)