Added the "file" type for forms and a handler for page not found(404)

To make the new handler work a new configuration is needed in the app/conf.lua:  
sailor.default_error404 = 'viewname' 
If 'viewname' == nil the render call is "ignored" and the default page is given to the client. (as a matter of fact i don't know why this happens, but it happens). 
After rendering, the server still gives a 404 answer to the client.

The other change, to form.lua, is pretty straight forward. 

If anyone knows how to split the pull request in two, please tell me.