Get video thumbnail before posting

[suth]

Getting a video thumbnail before posting can be a little messy when it comes to drafts and auto-saves, but support should be added.

[cohan]

The way I've tweaked it is to allow the clickable "Refresh image" option to work with drafts, but not the automatic grabber bit. It's not automatic but at least I can manually run it and know whether the image looks good before publishing