Attempt to parse BOM output file extension from BOM plugin header, and
append it to the "%O" argument in the BOM plugin command line.
Fixes#6943
(cherry picked from commit f54ab830f6)
* On Windows, a python command line must be python <script_path>/script.py
and not python <script_path>\script.py
* Fix also a incorrect warning message when a script is not found.
Wayne Stambaugh
Jon Evans
Graham Keeth
jean-pierre charras
Maciej Suminski