Multi-file and pre/post processing attachment support for Trac 0.13

[Mitar]

Hi!

I would like to turn your attention to those two tickets:

http://trac.edgewall.org/ticket/2141
http://trac.edgewall.org/ticket/6014

Would it be possible to include this in the 0.13 release? Currently
attachment system is really old-fashioned and it is hard to attach
multiple files (like multiple images for a gallery) at the same time.
Also preprocessing would be great to allow attaching compressed file
which would be extracted on the server and postprocessing to allow for
example images to be scaled or thumbnails automatically created.

I would propose that only hooks for pre/post processing would be made
so that plugins would then be possible for things I described above
and much more other will develop. Also current "replace existing file"
could be then implemented using such hooks.

For multi-file attachments HTML5 features could be used so that it
would be possible to select multiple files in a dialog box. Also
drag&drop would be useful. But I do not know how can this coexists
with Trac's strict XHTML.

This would greatly improve usefulness and user friendliness of the
attachment system.

Mitar