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.