Mime types are similar to file extensions. They are used to identify the type of information that a file contains. Occasionally people run into the problem of not being able have their special content downloaded, instead they get an ASCII stream or some other unusable data. To ensure the file is presented correctly or at least presents a option to download, add a MIME type so the client understands what type of data is being passed to them.  I run into this often when assisting people with Flash and Podcasts (MP4).

Example: The Flash Video Mime Type
Extension: .flv
MIME Type: flv-applicaton/octet-stream

Adding a Mime Type IIS 6
Launch IIS Manager either from the desktop icon or by using Start -> Run -> inetmgr
In the left column right-click on local computer and select properties.
Select the MIME Types tab.
Click New and enter the information.
In this case Associated Extension would be .flv
The MIME Type would be flv-application/octet-stream
Click ok
Restart the World Wide Web publishing service or issue an iisreset command.

MIME Types in IIS 7
Launch IIS Manager either from the desktop icon or by using Start -> Run -> inetmgr
Expand local computer and then expand sites.
Locate the site needing the mime type and select it.
Select the features view if it is not already visiable.
MIME types can be found under the IIS heading.
A new one can be added by selecting Add in the actions panel on the right.