There's an awesome video by @MichaelAubry about using Mesh files, MeshMixer, and Fusion 360 to get solid bodies.
https://www.youtube.com/watch?v=yMaih-vPNr4
Without looking the attached 21 MB file, my guess is that mesh is too dense for conversion. You can use the reduce tools in MeshMixer, to lower the triangle count.
Another good tip is to use Autodesk Momento to convert your triangle Mesh into a Quad Mesh. This will make for a smoother conversion to T-Spline (Sculpt) Bodies > to BREP.
Check out the below Autodesk University class for some further info on this topic.
http://au.autodesk.com/au-online/classes-on-demand/class-catalog/2015/fusion-360/cp9883#chapter=0
Thanks,
Nathan Chandler
Principal Specialist