The obvious answer might just be... don't.  The file name doesn't need to be in the card.

But let's say you did.  The Office variable mapping covers a lot of variables that you could map to, then leave your card controls on read-only (easy to use this for properties like Created, Modified, et al).  But MS-DOS Filename is not one of those variables (unless it is and that's just not documented in the Help files).

You could use the Default value to write a file name without the extension to an edit box, but...

• It doesn't update if the file name changes.  You can (should?) limit file name changes for most (all?) users anyway, but for files in a WIP state before they're approved/released it's often nice to be able to change the name of a file.
• While it works perfectly on files created by dragging from outside the vault (or files saved into the vault via save/save-as, or files made from a PDM template), when I use this property on a file created from RMB > New > Microsoft Excel Document, the property is written as "New Microsoft Excel Document" (and likewise for Word, PowerPoint, etc.  files) even when I change the name before doing an initial check-in.

So... how do you display this information?  The only other option I can think of is if you're actively using a serial number generator for a Document Number property (which my company is not).