Can not select download formatters

Hi,

tried to set up a file field to use the 'DownloadFile' formatters, but it does not work.

Four new formatters are listed in manage display settings. When choosing one, e.g. "Direct download file", a rotating throbber appears, but nothing else happens. The throbber keeps rotating, and when saving the display settings, it reverts to the default setting ("Generic file").

No errors are recorded at ./admin/reports/dblog.

I am using the 'bootstrap' theme and 'display suite'.