Well, spent the good amount of time trying to figure this out. See the configuration info below from my trac.ini file.
[mimeviewer]
max_preview_size = 262144
mime_map = text/x-dylan:dylan,text/x-idl:ice,text/x-ada:ads:adb,
php_path = php
pygments_default_style = trac
pygments_modes = text/x-csharp:csharp:7,text/plain:aspx-cs:7
tab_width = 4
treat_as_binary = application/octet-stream,application/pdf,application/postscript,application/rtf
Oh yeah, it helps if you actually install Pygments, too.