Short description
The new cache action gives the possibility to add cache files for an attachment of a page. This is done by storing data and meta data dependent to a key. This key is not guessable.
Sometimes you want to store more than one cache file per file. Therefore you can use a prefix.
Arnica for example uses a W for the webnails, a T for the thumbnails and a D for the date as prefix.
It will give confusion if an other program uses the same prefixes for different data.
We should find an easy ways to share these cache files and to protect the used prefix.
could add an arena parameter that defaults to cache_arena to all functions.