Exposed filters and contextual taxonomy description A view called "Music".

Fields: "title", "image"
Exposed filter: "category" (is a taxonomy-field = rock, jazz, salsa,...).
Every taxonomy-term has a description.

When the visitor for example, selects "rock" the results are filtered. 
So far, so good.

But i want also want that when a "category" is selected, the description of the selected term is displayed above the results. 

I think that could be done with an attachment and contextual filters, but i can't get it working.
You may use tokens (replacement patterns). If you add the term description in your fields and exclude it from display, you will have access to [description] token in a Global: Text area, for example, added in the view header. Then, if you check Use replacement tokens from the first row and set your exposed form style on Input required, when a category is selected, results will be filtered and the term description related to this category will be displayed in the header.

EDIT : for Drupal 8 (sorry, still more a "7 guy" ^^), the token follows Twig syntax : {{description_value}}