From the RFC (3023), under section 3, XML Media Types:
If an XML document — that is, the unprocessed, source XML document
— is readable by casual users, text/xml is preferable to
application/xml. MIME user agents (and web user agents) that do not
have explicit support for text/xml will treat it as text/plain, for
example, by displaying the XML MIME entity as plain text.
Application/xml is preferable when the XML MIME entity is unreadable
by casual users.
(emphasis mine)