You have a custom MIME filter that meets the following conditions:
The MIME filter is expected to be registered and unregistered dynamically in a WebBrowser control host application.
The WebBrowser control browses to local content by using a file protocol.
When you use this custom MIME filter in Windows Internet Explorer 9, you may notice that the MIME filter is disabled and is not invoked.
This issue occurs when the response that contains the MIME type is returned, and Internet Explorer cannot find a registered MIME filter for the MIME type.
Note If you use Windows Internet Explorer 8, the custom MIME filter is always invoked.
To resolve this problem, install this update. After you install this update, you must enable the hotfix.
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
To enable the hotfix, follow these steps:
Start Registry Editor. To do this, click Start, click Run, type regedit, and then click OK.
Locate and then click to select the following registry subkey.