Outlook Web App now integrates the Office Web Apps into the attachment previewing experience for Word documents, Excel spreadsheets, and PowerPoint presentations.
When Office Web App attachment preview is enabled, users see an Open In Browser link next to the Office Web App document type in Outlook Web App. By clicking on the link...
...the user can open the attached document using the Office Web Apps to get a high-fidelity preview of the Office document in a web browser window.
Disabling the Feature
By default, this feature is enabled in Exchange Online. It can be disabled using the Remote PowerShell command Set-OwaMailboxPolicy Default -WacViewingOnPublicComputersEnabled $false –WacViewingOnPrivateComputersEnabled $false
Preventing Attachment Downloads
Administrators can block users from downloading attachments in Outlook Web App. This helps prevent users from accidentally leaving content on an unsecure machine, such as an Internet kiosk. Attachment download settings in Outlook Web App are managed through Remote PowerShell (Set-OwaMailboxPolicy Default -DirectFileAccessOnPublicComputersEnabled $false).