Allow the 'accept' file upload attribute to be set for artefact chooser

[Robert Lyon]

Now that bug #1262928 is done we should expand this to deal with items uploaded by the artefact chooser.

This is a bit more tricky as the chooser can be used for uploading different things depending on where it is called

What we would need to do is:

1) Pass to the artefactchooser an array to indicate what types of acceptable files can be chosen

2) To keep a list of acceptable types so that we can add/edit in one place rather than having to update all the forms (so need to tweak the existing csv ones)

This way if a user is trying to add an image block they can filter the list to valid image types when uploading etc.