Mutt's mime_lookup list specifies a list of mime-types that should not be treated according to their mailcap entry. This option is designed to deal with binary types such as application/octet-stream. When an attachment's mime-type is listed in mime_lookup, then the extension of the filename will be compared to the list of extensions in the mime.types file. The mime-type associated with this extension will then be used to process the attachment according to the rules in the mailcap file and according to any other configuration options (such as auto_view) specified. Common usage would be:
mime-lookup application/octet-stream application/X-Lotus-Manuscript
In addition, the unmime_lookup command may be used to disable this feature for any particular mime-type if it had been set, for example, in a global muttrc.