works fine for me with the 700+ file format specification
gtk3-scrolling.webm (video/webm), 1.08 MB, created by Caolán McNamara on 2016-03-17 15:21:21 UTC
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Creator: Caolán McNamara
Created: 2016-03-17 15:21:21 UTC
Size: 1.08 MB

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Attachments on bug 98298: 123662