Office document (such as .docx) thumbnails or previews are not displayed in your FAST Search results for documents from SharePoint sites extended with Claims Based Authentication.
The symptom is a known limitation with document previews. In this scenario, the FAST Search web parts do not have a method to pass the claim token within the wacproxy redirect request between the SharePoint WFE and the backend WAC server, and therefore access is denied.
To confirm this problem, one can utilize a Web Traffic Analyzer and check the response provided to wacproxy.ashx requests.
If Claims Authentication is being utilized the wacproxy.ashx response will contain the following:
parent.window['DelegateFn_SRB_g_3a30f0ed_aa28_455d_8bf4_099fc3b9a46d_1']('ERROR:603:WebException - The remote server returned an error: (401) Unauthorized.');
However, while using Classic or Kerberos Authentication, the wacproxy.ashx response will look similar to the following:
In order to resolve this problem one must remove the Claims Based Authentication from the SharePoint Sites in question and use Classic or Kerberos Authentication, as these Authentication methods are fully supported with Document Thumbnails and Previews.
This article describes a known limitation of the FAST Search web parts provided with SharePoint 2010. The Microsoft SharePoint Product Group is fully aware of the issue and considering implementation of a resolution in future versions of the product. Microsoft will post new information in this Microsoft Knowledge Base Article as it becomes available.