Allow selection of file grouping method

[Floppy]

It might be good to provide some options for file grouping which people can choose between. How do we know what people want in every case? @rukongai in #1701 said:

It'd be nice if files could be broken out by filetype. Images grouped together, lys, stl, etc.

Originally posted by @Floppy in https://github.com/manyfold3d/manyfold/issues/50#issuecomment-1933999203