From Microformats

Content Type

AKA:

• HTTP Content-Type (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.17) - this is the preferred terminology, especially as it emphasizes the content aspect.
• MIME (http://en.wikipedia.org/wiki/MIME) type. This terminology is deprecated.
• media type (http://en.wikipedia.org/wiki/Internet_media_type). Recommend AGAINST using the term "media type" to refer to content-type as it is overloaded, and often (even more often among web design circles) used to refer to CSS media types (http://www.w3.org/TR/CSS21/media.html).

In microformats, as elsewhere, it is good practice to use a content-type attribute when linking to audio files, PDF files, and other attachments.