Here's a list of file types recognized by Kindle Touch:
Documents: Kindle (.AZW3, .AZW, .AZW1). Text (.TXT), Unprotected Mobipocket (.MOBI, .PRC), .PDF
Audible:Audible (.AA, .AAX)
Music: MP3 (.MP3)
Source: Amazon.com Help: Transferring, Downloading, and Sending Files to...