Sometimes attachments in tickets offer both options (buttons) to download and view attachments and sometimes the "View" button is missing. The view option is available only for the attachments of the following content-types:
audio/* video/* image/* except svg
So essentially you can "view" without first downloading the attachment all audio files, video files and image files with the exception of the SVG format which cannot be viewed directly.
The following content types are viewable by Google viewer:
application/msword application/vnd.* application/pdf
The attachments which can only be downloaded have content type set to application/octet-stream.
The content-type header for attachments is set by the application that sends the email and LiveAgent just displays the attachment options accordingly - displays the "View" option or not.