On a server with a back end of Exchange 2003 on IIS6 the relevant MIME types for the new Office 2007 formats may be unregistered. To resolve this the following should be performed :

Open IIS, Right Click on Server, select properties. In MIME types add the following -

.xlsx  application/vnd.openxmlformats-officedocument.spreadsheetml.sheet 
.xltx  application/vnd.openxmlformats-officedocument.spreadsheetml.template 
.potx  application/vnd.openxmlformats-officedocument.presentationml.template 
.ppsx  application/vnd.openxmlformats-officedocument.presentationml.slideshow 
.pptx  application/vnd.openxmlformats-officedocument.presentationml.presentation 
.sldx  application/vnd.openxmlformats-officedocument.presentationml.slide 
.docx  application/vnd.openxmlformats-officedocument.wordprocessingml.document 
.dotx  application/vnd.openxmlformats-officedocument.wordprocessingml.template 
.xlam  application/vnd.ms-excel.addin.macroEnabled.12 
.xlsb  application/vnd.ms-excel.sheet.binary.macroEnabled.12 

In addition these should also be registered in Exchange System Manager as follows -

Open ESM, Expand Global Settings, Right click Internet Message Formats then hit properties.
Add the above mime types (Mime type first, then extension in ESM).

You may need to restart IIS and the Exchange information store for the changes to take effect.

In addition the client user may need to add the OWA server address to their list of trusted sites if IE8 or above it being used.

Andy.