The decision hasn't been made yet, but the new file manager should arrive in Ubuntu 14.10
Ubuntu is still using a lot of GNOME applications, including the file manager Nautilus (now Files), but that will change soon enough.It seems that the way GNOME developers are working on Nautilus is not exactly helping Canonical. Ubuntu developers are unhappy with the some features that have gone missing from the file manager and are discussing making their own. More precisely, they will converge the one already used on the phone and tablet.
"With the planned switch to unity8 in 14.10 it is most likely that we will also start using the converged QML apps that are developed today. With all the complaints and unhappiness about Nautilus upstream ripping out things like dual pane and other beloved and helpful features I expect we can do better,” said Ubuntu developer, Oliver Grawert.
Canonical plans to converge the platforms starting with Ubuntu 14.10, which will be released later this year in October. This means that they will also probably take the file manager and make it available on all the platforms.
For now, Ubuntu developers are trying to get a feel from the community and understand what features are needed. More information about this major change should become available soon.
UPDATE: It has been pointed out that this discussion is in its preliminary stages and that there are a lot of things to figure out before this becomes a reality.
I have to emphasize the fact that getting the File Manager instead of Nautilus in Ubuntu 14.10 is a proposition for developers, not a promise.
Also, the word "soon" in the title is used to describe a period of time that emcompases more than one development cycle. That should have been made clearer.