It is a very common request to have model and drawing match except for the extension - repeated like clockwork very often. Not addressed by PTC.
With auto-number (system default), this is not provided.
With manual number, users can type in same for both; system will fill in .prt, .asm, .drw, etc. Don't have users type in the extension.
Enter: 123456 when creating prt, drw
System result will be: 123456.prt, and 123456.drw
note: For Rename, user needs to keep the extension
note: If you use WTParts, the WTPart should have the same number with no extension.
It is possible to have customization (relatively complex) that re-assigns the Number of the WTPart and related CAD.
Bottom line - choosing to auto-number or not is critical and using manual is currently the only straight-forward and simple way to have them match.
Very likely should add a human check (or could be done by code) to verify the match without the extensions (e.g. .prt, .drw). Can also use a query builder report to find mismatches.