I have files which I need to store for 10 years or for ever. If it's for 10 years, file storage will end 12/31 ten years later. Is there a formula which gives me based on the "end"-date of the content of the file (say i.e. October 10, 2004) either an expiry date 12/31/2014 (it must be 12/31...)or - if its for ever - a date like 12/31/2999??

Try the following:

= Date (12; 31; Year(End_Date) + 10)