About content types and compression

In general, some Web servers, when responding to a request, do not accurately provide the content type in the response header. For example, a response may include a Microsoft Office PowerPoint® 2007 (.pptx) file, but the response header may indicate that the content is plain text. When an Internet Explorer client receives this type of response in compressed format, it cannot interpret the response, and the user sees meaningless characters on the monitor. If the response is received uncompressed, Internet Explorer can interpret it, and the user can open and view the content. However, if the client requests compression and the Web server replies with uncompressed content, Microsoft Forefront Threat Management Gateway compresses the content, and Internet Explorer cannot interpret it. In this case, the client must request uncompressed content (by changing the compression setting in the browser) in order to view it.

Compressing content types

When HTTP compression is enabled, the default setting is to compress the following content types:

  • HTML Documents
  • Text

However, you can specify which content types are compressed. You have the option of specifying the content types that are excluded (all others are included) or the ones that are included (all others are excluded). Then select specific content types to be excluded or included. You can create new content types in the Firewall Policy task pane, on the Toolbox tab, as described in Create a content type.

The following content types cannot be compressed because they are already compressed or cannot be compressed because they are provided as a stream:

  • video
  • audio
  • application/x-tar
  • x-world/x-vrml
  • application/zip
  • application/x-gzip
  • application/x-zip-compressed
  • application/x-compress
  • application/x-compressed
  • application/x-spoon