Office 365 - What is the reason I can’t open or view attachments in Outlook Web App?

By default, Outlook Web App blocks attachments that have the file name extensions such as .mat, .exe, .bat, .chm, .vb file, etc.

When Office 365 users try to open or view attachments with these extensions in email messages with Outlook Web App, they experience the following symptoms:

• In Office 365, the attachment isn't displayed in the message. Instead, a generic placeholder, such as “1 Attachment” is displayed, and this can't be opened.
  
  
• When users try to view attachments in a pre-upgrade installation of Office 365, they receive the following error message:
  
  Access to attachments has been blocked. Blocked attachments: <FileName>.

Users are advised contact the sender to compress the files that they intend to send (for example, as a .zip file) and then send the compressed files as attachments. Details please refer to Microsoft's support page at http://support.microsoft.com/kb/2852113.