formats/format_wav: enhancing log message "Not a wav file" to be clear on what is supported

Modifying the log message to be more specific as to what is supported. Specifically it seems format_wav supports only PCM encoded versions with a lower-case '.wav' extension.

(closes issues ASTERISK-22310)
Reported by: Jim Credland
Review: https://reviewboard.asterisk.org/r/3188/
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Merged revisions 407511 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 407512 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@407513 f38db490-d61c-443f-a65b-d21fe96a405b