FULL PRODUCT VERSION :
found in Java 7 sources, present in Java 6/7

ADDITIONAL OS VERSION INFORMATION :
probably any (source bug)

A DESCRIPTION OF THE PROBLEM :
trying to use guessContentTypeFromStream on any stream containing Microsoft RIFF container other than the actual WAV results in identifying the file as WAV, Per source,
if (c1 == 'R' && c2 == 'I' && c3 == 'F' && c4 == 'F') {
/* I don't know if this is official but evidence
* suggests that .wav files start with "RIFF" - brown
*/
return "audio/x-wav";
}

- obviously brown is wrong, see links below for detailed rationale; while WAV files do have RIFF signature, 'RIFF' 4CC only signifies a RIFF container has been found. Any RIFF (WAV/AVI/RMI/CDR etc) has RIFF 4CC at header start; it's necessary to peek into next bytes to be able to accurately identify the file type.

http://en.wikipedia.org/wiki/Resource_Interchange_File_Format
http://oreilly.com/www/centers/gff/formats/micriff/

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
try to use guessContentTypeFromStream on any stream containing Microsoft RIFF container

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
identify the container based on first encountered chunk type in RIFF file
ACTUAL -
always erroneously identified RIFF as WAV

ERROR MESSAGES/STACK TRACES THAT OCCUR :
no error reported by VM

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
replacing
if (c1 == 'R' && c2 == 'I' && c3 == 'F' && c4 == 'F') {
/* I don't know if this is official but evidence
* suggests that .wav files start with "RIFF" - brown
*/
return "audio/x-wav";
}
with
if (c1 == 'R' && c2 == 'I' && c3 == 'F' && c4 == 'F') { // Microsoft RIFF container
              if (c9 == 'W' && c10 == 'A' && c11 == 'V' && c12 == 'E' )
return "audio/x-wav";
              if (c9 == 'R' && c10 == 'M' && c11 == 'I' && c11 == 'D' )
                return "audio/mid";
             if (c9 == 'A' && c10 == 'V' && c11 == 'I' && c11 == ' ' )
                return "video/avi";
             // and so on, for other RIFF formats such as CDR, ANI, DLS, WebP etc
            }
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Don't use guessContentTypeFromStream and use own method designed for that case.