All reports of doc-base-file-unknown-format for the archive. The extended description of this tag is:
The Format field in this doc-base control file declares a format that is not supported. Recognized formats are "HTML", "Text", "PDF", "PostScript", "Info", "DVI", and "DebianDoc-SGML" (case-insensitive).
Refer to Debian doc-base Manual section 18.104.22.168 (Format sections) for details.
This tag has not been emitted in any package tested by Lintian.