This is an addon that makes it easy to discover MIME types. The basic implementation (contained in this addon) uses the Apache Mime Types definitions found here:
http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/conf/mime.types?view=co
Other MIME-type discovery implementations (e.g. via libMagic, libAV) etc can easily be created by implementing BaseMediaTypeProvider). See https://github.com/bakercp/ofxLibmagic for an example.