How to Preview Attachments in Standard Gmail
In the standard web version of Gmail, you can open attachments using the Google Docs Viewer, the same pop-up window used on Google Drive.
While viewing a message in Gmail, click on the preview of the attachment to open it.
If the file type is supported, it will open in the Docs Viewer.
Attachments Supported in Google Docs Viewer
Gmail allows you to send almost any file type as an attachment, but only certain files can be previewed within your browser.
Supported Microsoft File Types
Select View as HTML under the attached file.
If supported, the file will open in a separate window.
Attachments Supported for Viewing as HTML in Basic View
The following file types are supported for viewing as HTML in Gmail Basic:
