The Helm 4 MIME types icon is used to add custom MIME types to you
domain. They are used to tell the domain how to handle different kinds
of content, for example flash .flv files.
For more information on what MIME types are is located here.
To add a custom MIME type begin by navigating to a domain and
clicking on the MIME Types icon, this will display a list of custom
MIME types for this domain.

Select 'Add', then input the file extension without the leading "." and how you want the information handled.

After you add the MIME type you will be returned to the list of MIME
types, in this example we have added the 'Flash FLV' MIME type.
