Returns or sets a String value representing the name
of the file containing the handler function. The full path should be
specified. The type of file depends on the Command.Language,
for example VBScript commands are implemented in .vbs script files, and
C++ commands are implemented in .dll or .so files.
Note: Changing the filename automatically updates the Command.Language
from the file extension if possible.
This property should not be set if Command.Code is used to
provide the implementation. It is also not necessary to specify this for a
self-installed plug-in.