Based on the version of the control panel you're using the procedure will vary:

              PLESK 8.2.0 for Windows (web18)
      

1. Login to PLESK
2. Click Your Domain
3. Click the Web Directory Icon. From this screen you can add your MIME Type

PLESK 7.5.x for Windows (web16, web14, web12, web10)

1. Login to PLESK
2. Click Your Domain
3. Click the Web Directory Icon. From this screen you can add your MIME Type

PLESK 7.0.x for Windows (web6.3essnentials.com, web8.3essentials.com)

1. Login to PLESK
2. Click your Domain
3. Click the MIME Icon

Below are some currently documented MIME Types. The format is Extension ==> Content. Enter these in the appropriate fields in PLESK. If the content type is not listed in the drop down menu, use the custom setting from the drop down menu.

Flash Video via Flash Player
Embedded video (requires SWF)
Progressive download (requires SWF and FLV)
Streaming video (requires SWF and FLV)

.flv ==> video/x-flv
.swf ==> application/x-shockwave-flash

For programming WAP Pages
.wml ==> text/vnd.wap.wml
.wbmp ==> image/vnd.wap.wbmp
.wmls ==> text/vnd.wap.wmlscript

For Silverlight
.xap==>application/x-silverlight-app