This ensures that destination files for HTML posts, pages and
collections always include the proper file extension (as defined by
output_ext) regardless of permalink structure. This allows for URLs
that contain no extension or trailing slash to still result in proper
destination files with .html extensions.
Because this change relies so heavily on output_ext accurately
identifying the extension of the destination file, this change also
removes the feature test that tested support for permalinks with a .htm
extension. In order to support alternate file extensions, a future
patch or plugin will need to modify the output_ext value, at which point
everything else should work as expected.
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