Can I add or change the file types Drive Bender uses in its file count?

Yes, the file types extension are set via the registry in "HKEY_LOCAL_MACHINE\SOFTWARE\Division-M\Drive Bender\File Types"
Note, you cannot add new categories, you can only edit the available file types for the existing categories. 

Each sub key (i.e. Document, Music etc) contain an "Extension", this contains a comma delimited string of the extension. For example the Documents list looks like "DOC,DOCX,LOG,MSG,PAGES,RTF,TXT,WPD,WPS".

You are free to edit this list providing the following rules are observed.
1) There are no spaces.
2) The list must remain all caps.
3) The list must be in alphanumerical order.
4) The extension's period is omitted (i.e. do not use ".BAT", instead it must be "BAT"). 

Once you have modified the list, you will need to restart the machine. After the machine restart, Drive Bender will automatically perform a file system health check (may take some time to complete). Once done you should see the updated file count.