- instead of registering a callback, a file sending manager now register itself and must
implement some well known method (`fileSend`, `canHandleFileSend`) and optionally a
`name` attribute
- `utils.asDeferred` is now used for callbacks, so all type of methods including
coroutines can be used.
- feature checking is now handled by `canHandleFileSend` method instead of simple
namespace check, this allows to use a method when namespace can't be checked (this is
the case when a file is sent to a bare jid with jingle)
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