If you cannot view an attachment in the web mail interface, it is mostly that the file is coming from Outlook. You can view the RAW Content of the email and search for “winmail.dat”. If it is present then the file was sent using either Rich Text or HTML format.
To resolve this issue:
- Open Outlook
- Click Tools
- Click Options
- Click Mail Format tab
- In the Message Format group
- Set Compose in this message format to “Plain Text”
- Click Apply
- Click OK
Here is a Microsoft article for reference:
http://support.microsoft.com/kb/278061
