Rosen 410X Owner's Manual page 12

File Extensions
Always add a file extension " .MP3" or ".mp3" to an MP3 file
by using single-byte letters. If you add a different file exten-
sion, or forget to add the file extension " .MP3" or ".mp3," the
file cannot be played. In addition, if you use upper case and
lower case letters (capital and small letters) mixed together for
file extensions, normal play may not be possible.
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