Assuming it's a supported attachment type (
KB20541-Supported attachment types in BlackBerry Enterprise Server 5.0) and the attachment type is allowed in your environment, make sure the BlackBerry Attachment Service is running. If it's running, restart it to see if this will resolve the issue. You can also test the attachment service in the Server Configuration Tool (Start --> All Programs --> BlackBerry Enterprise Server --> BlackBerry Configuration Tool). Select the Test Attachment tab, fill in the fields, and select the attachment to see if the Attachment Service has the distillers to convert the attachment. It'll tell you it's successful if it's able to convert the attachment.