To add to this, it is possible to preserve the actual extension, but at the cost of losing the ability to rename the file; though it could be argued that this shouldn't be allowed anyway from a document control perspective.
To do this, go to DC30 and tick the "NoRename" option for the given "Workstation Application" (i.e. Appl). Save changes and you're golden. 
As a reference, here is the F1 documentation on the field:
Indicator: no renaming of temporary file
When you display an original application file that is stored in a secure storage area, this indicator determines whether the file can be renamed.
Use
To display an original application file that is stored in the SAP database, a vault, or an archive, the system creates a copy.
- In the standard SAP System, the copy is assigned a name according to the naming convention defined in the program.
- Some viewing programs can only find an original application file if the copy of the file has the name that the file had when it was saved to a secure storage area. In this case, the naming convention links the redline file to the viewer file.
If you set this indicator, the copy is assigned the name that the original application file had when it was saved to the secure storage area.
Procedure
Only set this indicator if your viewer application does not allow renaming of the file.