To specify which file extension to use in links to other HTML files:
; FileSuffix = suffix used in xrefs for HTML: .htm (default), .html
Whether to set FileSuffix=html depends on the convention used on your site. Use htm (the default) unless you have a good reason to specify the full original html extension.
§13.2.4 Specifying a different output file extension
§19.6.1 Identifying HTML link destinations with FileIDs