Do away with antiquated file formats and export your animated meshes in glTF format for better support and potentially smaller file sizes. The glTF file format has much better support than .b3d and can be imported/exported from many 3d modeling programs. This will make editing models/animations from mods easier, as you won't need the source files, although having the source files is always best.

Below are the settings I'm using for exporting a static(non-animated) mesh.

export settings for static meshes.

 

These are the settings I'm using for animated meshes. Keep in mind you do need to change the framerate to a custom setting as mentioned in the video for the animation to display correctly within Luanti.

export settings for animated meshes.