It seems like there’s hidden rules on how to correctly get something to embed properly from giphy, imgur, or redgifs. I have some control over it, but most of the power lies in the Discourse software or the linked websites themselves.
Here, I tried plugging in some links to see what works. Note that Discourse does some background processing, so an embed might appear to work initially, but ~10 minutes later your embed could break.
This is also dependent on some technical details of the website you’re linking to, so they could change their behavior in the future. Also the file size of your link could impact preview as well. Overall, it seems that problems are most common when linking directly to a .gif file
If problems keep persisting, I could probably allow for iframe embedding from these sites.
Not allowing edits on this topic for now
gif link (downloaded locally, so this will not preview with larger files)
media social (downloaded locally, so this will not preview with larger files)
media small (downloaded locally, so this will not preview with larger files)
copied link, which is the same as the page url
link to video location source
imgur link without /gallery/. I think this is how they treat nsfw links?
above link to video source
above link to gif
direct gif link (small) (downloaded locally, so this will not preview with larger files)
direct gif link (large)
right click copy video location