I created some MP3 files containing a talking clock to use in my phone’s morning alarms and called them things like 6:30.mp3. When I tried to add them as alarm tones, I saw:
Sorry, the player does not support this type of audio file.
As they were only a handful of small files, freshly generated with LAME, and transferred via Dropbox, file corruption seemed rather unlikely. A little searching turned up a comment about filenames starting with digits causing this error (as is often the case for track numbers for tracks transferred from CDs). Removing the leading digits - and while I was at it, the colons - caused the problem to go away.