Avoid rebuilding mime type regexp for every file.

Check for object equality never worked (not possible without some
external utility).
o IS mOxie within the closure, no need to reference it explicitly.
Do not build up recursions stack, when adding files to the queue. Close #897.

There's a limit on that and it is different for every browser.
Trigger FileFiltered after file is filtered, but before it is added to the queue.

A way to add the file to the visual queue early.
UI: Make use of FileFiltered event to display the file as soon as it is known to be eligible for the queue.
Support for multiple file pickers and drop zones.