MIME policies control the access for specific MIME types. For example, a .pdf file usually have a mime type of application/pdf. MIME types are defined under the Components menu option. Any file contained within and email that fails the MIME policy will fail the scanning check.
Deny by Default
If enabled, any MIME type found that is not listed within the MIME policy will fail the scanning check. If disabled, a non-match will be allowed to pass.