Unknown file format, similar to or containing MSBT files. There is no identifiable magic number.
According to a 3DLand MSBT Editor , UMSBT files have to be unpacked into MSBT files. Further testing on more UMSBT files (like those in Animal Crossing: New Leaf ) needs to be done to make sure that's correct for all UMSBT files.
Unpacking the UMSBT file then opening it seems to break the files, with lots of <Null> tags added. It's possible that Animal Crossing's UMSBT files contain more data, such as when an appropriate script should be read.