Default Media File Store virtual methods

[deanmarcussen]

Recently I had to override one method on the DefaultMediaFileStore, which meant copying the entire code over, instead of inheriting, and tweaking the bit I needed.

Think it makes sense to make the methods virtual, to make changing the behaviour a little easier.

[hishamco]

Does make all the methods virtual make sense? or there some of them that are really could be overriden