Purge cache doesn't work for me (Moodle 2.5.4+)
The downloaded file seems not recognized by the OS.
For example in Android, the PDF file has its unique icon. But when I browse to open download folder via default "Downloads" app, the PDF is with an blank icon.
Summary of problem:
iOS - "undefined" word shown. Refresh safari on same URL helps.
Android - File is downloaded but a toast message says "Can't open file".