No support for LDIF files at this point.
The interface for defining field mappings in CSV files is normally the first line of the file. For example:
At some point in the future we'll probably have more options for importing (LDIF, vcards, a specific CSV flavor (thunderbird, yahoo, etc)), but at this point we require the mappings as the first line of the file, which we use to map into our contact model.
"First Name","Last Name","E-mail Address"
We've successfully imported Thunderbird, Yahoo, Google, and various forms of Outlook CSV files.
As KevinH pointed out, the best way to see what fields we natively support is to completely populate a contact within Zimbra, and do a CSV export.