idx

No, because I do not know what .idx has to do  with a 3d cad

Google:

The file extension .idx usually denotes an index. Such is created by various programs and uses. Do you want to open an IDX file, you must use for each matching application.

Most references an IDX file to a database, because the index is intended to speed up the search in a database. You can open a file IDX usually only when you access the program that the IDX file created.

Which program should open your .idx file?

Take a look at this discussion ECAD-MCAD collaboration

This is a thesis on ECAD-MCAD collaboration https://www.theseus.fi/bitstream/handle/10024/72206/Valkky_Markus.pdf?sequence=1

It talks also about IDX format