Subject: app_mixmonitor

If the 'filename' argument to MixMonitor() ended with '.wav49,'
Asterisk would silently convert the extension to '.WAV' when opening
the file for writing. This caused the MIXMONITOR_FILENAME variable to
reference the wrong file. The MIXMONITOR_FILENAME variable will now
reflect the name of the file that Asterisk actually used instead of
the filename that was passed to the application.
