Hi
... no the problem is much simpler. Filenames with blanks in it will result in problems because the mhmat file is simply parsed and split into "words".
Since that is no rocket-science, here the full information:
yes, you can always rename the file and then change the entry in the .mhmat file. Only when you change the name of a .mhmat file itself, edit the .mhclo (where it is referenced) also.
The files with ".npz" and ".mhpxy" are the only binary ones (except from the textures of course), but they are created by MakeHuman itself. As long as a .mhclo and .obj is there, you can throw them away, they will be re-created (it is a bit like compiling). But there is no reason for throwing them away in this case. It is only referenced in the .mhmat file.
Since these are user assets we cannot fix it, sorry.
Of course it would be much better, when the uploader names a file <my_asset>.jpg or <my_asset>_diffuse.png
We also had full pathnames like c:\myhomepath btw. which always will result in trouble, not only for the Linux and Apple users, but for all those not using the exact same path