The email industry estimates that 80% of email data is stored via attachments. Email standards only allow text. Attachments must be encoded with MIME/base64 format.
Outlook XP: %USERPROFILE%\Local Settings\ApplicationData\Microsoft\Outlook Win7/8/10: %USERPROFILE%\AppData\Local\Microsoft\Outlook
MS Outlook data files found in these locations include OST and PST files. One should also check the OLK and Content. Outlook folder, which might roam depending on the specific version of Outlook used. For more information on where to find the OLK folder this link has a handy chart: LINK