* [MM-10813] Design update for Post attachments
Fixes https://github.com/mattermost/mattermost-server/issues/12841
* Updated visual treatment for Post attachments (images, documents, videos)
* Grouped image attachments, expanding to fit Post width dimension
* Itemized listing of non-image attachments
* Special handling of "small" image attachments (<48 point width/height)
* Set attachment post width to max width of portrait orientation
Accounts for post display offset and extra spacing used for rendering post replies.
* Use available Post real estate: flex: 1 instead of width 100%
* Image spacing responsibility: AttachmentList -> AttachmentImage
* Fit download progress circle correctly over new attachment icons
* Layers progress circle over the icon, rather than under.
* Uses offset constant as far as possible, rather than fixed point spaces.
* Refactor props and 'more' counting for image file attachment(s)
* Implement conditional gutter between image attachments
Flex's `justifyContent: space-between` won't work in this case because of the use of `absoluteFill` and `paddingBottom: 100%` in the box placeholder for an image attachment to auto-fill *all* available flex space.
* Additional snapshots for Post file attachment scenarios
* Use new 'text' icon for text files (.txt, .rtf)
Depends on https://github.com/mattermost/mattermost-redux/pull/979
Even without the change to mattermost-redux above, text files will default to pre-existing "code" icon.
* Set file attachment icon background to theme
Default to transparent. Override if explicitly specified.
* Treat animated GIFs as images when auto-adjusting attachment width
* Fix images layout, progressive image margins, and gallery for images and videos
* fix on iPad (splitview, permanent sidebar) card types and image sizes
* Add all files back to the gallery
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