How to configure static content cache per folder and extension in IIS7?

You can set specific cache-headers for a whole folder in either your root web.config:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <!-- Note the use of the 'location' tag to specify which 
       folder this applies to-->
  <location path="images">
    <system.webServer>
      <staticContent>
        <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="00:00:15" />
      </staticContent>
    </system.webServer>
  </location>
</configuration>

Or you can specify these in a web.config file in the content folder:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <staticContent>
      <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="00:00:15" />
    </staticContent>
  </system.webServer>
</configuration>

I'm not aware of a built in mechanism to target specific file types.

You can do it on a per file basis. Use the path attribute to include the filename

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <location path="YourFileNameHere.xml">
        <system.webServer>
            <staticContent>
                <clientCache cacheControlMode="DisableCache" />
            </staticContent>
        </system.webServer>
    </location>
</configuration>