The attachment plugin's Insert Links button currently only knows
how to insert plain wikilinks and img directives (for images).

[[wishlist]]: Generalize this, so a plugin can cause arbitrary text
to be inserted for a particular file. --[[Joey]] 

Design:

Add an insertlinks hook. Each plugin using the hook would be called,
and passed the filename of the attachment. If it knows how to handle
the file type, it returns a the text that should be inserted on the page.
If not, it returns undef, and the next plugin is tried.

This would mean writing plugins in order to handle links for 
special kinds of attachments. To avoid that for simple stuff,
a fallback plugin could run last and look for a template
named like `templates/embed_$extension`, and insert a directive like:

	\[[!template id=embed_vp8 file=my_movie.vp8]]

Then to handle a new file type, a user could just make a template
that expands to some relevant html. In the example above,
`templates/embed_vp8` could make a html5 video tag, possibly with some
flash fallback code even.
