Google Message Security for Google Apps Administration Guide - Manual (Page 273)

Attachment Manager 273
Note that binary scanning now identifies Microsoft Office 2007 file types. If
binary scanning cannot determine the file type of an attachment, Attachment
Manager uses extension scanning as a fallback method.
Note: In some cases, binary scanning does not distinguish among some
related file types, if they are generated by the same application. For example,
binary scanning treats .pps (PowerPoint SlideShow) files as .ppt (PowerPoint)
files.
• Compressed File Scanning: Attachment Manager automatically scans and
identifies files inside compressed file attachments, such as .zip and .rar. files.
This serves as an effective anti-virus tool since virus attacks are often
delivered as executables files hidden in compressed files.
For example, if a message has a .zip file attached, Attachment Manager can
apply filter rules to the .zip file and all the files contained in the .zip file. This
Attachment Manager feature offers additional protection against zero-hour
threats, such as virus-infected .exe files hidden in .zip files, and finer
granularity of control over the file type filters.
Attachment Manager can also identify files in certain types of encrypted
compressed files. If the file names in the archive are unencrypted, Attachment
Manager will identify the types of files. Malicious senders typically use
common compression tools to password-protect the contents of the archive,
but not the file name information.