Bug 1422891, mentions that Chrome's media recorder records to Matroska (which is a superset of WebM). FYI, the 'intentional' link may be to the wrong bug.
Bug 1551765 Comment 7 Edit History
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Bug 1422891, mentions that Chrome's media recorder records to Matroska (which is a superset of WebM).
Bug 1422891, mentions that Chrome's media recorder records to Matroska (which is a superset of WebM). Perhaps the file was renamed?