I evaluated some possibilites.
My favorite idea is to provide a new platform (e.g. under up.metin2dev.org) which uses the forum accounts.
You can upload to it any files you want to and every user gets 5GB of space (except blue rank users who get 50GB of space).
Every file will then be stored in an OpenStack Object Storage. This will be okay for the price, also I will implement then a donation button for this platform.
For make the costs a little bit more transparent I can say that this will costs me 1 cent / GB / month for storage and 1 cent / GB download traffic.
Is this a good idea?