“Den angivne sti blev ikke fundet” and “C:\Dokumenter\Modtagne filer”, do you konw what these mean in English?

Also look in the avast log viewer for any errors, you can get to the log viewer by right clicking the “A” ball in your task bar (next to your system clock) and selecting “avast! Log Viewer”.

EDIT: Just made sure its not a bug, got a mate to send me a .gif and a .jpeg, transfer went fine.
Are you sure its not just them files, have you tryed getting someone to send you a different file of the same type (.jpg/.gif)

–lee