How can I set what available types that can be dropped in the contentarea?
In EPiServer 7 I used OpenWaves Extended ContentArea Editor plugin to handle this. But that plugin dont't seem to work in 7.5. I read that in 7.5 there will be an attribute that you can set this with.
Yes, that´s correct. Take a look at this post: Restricting the allowed types in a content area and Simplifying allowed content types in editor descriptor.