prevent race condition when attempting to cancel a file upload

spec/features/issues_spec.rb

@@ -585,9 +585,11 @@ describe 'Issues' do
       end
 
       it "cancels a file upload correctly" do
-        dropzone_file([Rails.root.join('spec', 'fixtures', 'dk.png')], 0, false)
+        slow_requests do
+          dropzone_file([Rails.root.join('spec', 'fixtures', 'dk.png')], 0, false)
 
-        click_button 'Cancel'
+          click_button 'Cancel'
+        end
 
         expect(page).to have_button('Attach a file')
         expect(page).not_to have_button('Cancel')