MIME Types are used in MIME Policies. They determine the "kind" of file by examining the properties of the file. They are able to determine type regardless of the file extension. For example, if someone renames program.exe to program.zip, Mailborder will still be able to determine that program.zip is actually an executable file.
If you have files being stopped due to MIME failures, examine the Scan Report of the email.
System > Maillog > open the email > Scan Report
You can scroll through the report and see the MIME information for any attachment.
You may use wildcards to add MIME types. For example:
Will match all Open Office document formats.