Use thumbnailUrl/posterUrl (and the image url itself for image-type
attachments) inside the 52x52 box of the file/document bubble, falling
back to the file-type icon when no preview is available.
Also tune the deep-link scroll-mt offset (82px / 98px) so a targeted
bubble lands just below the sticky header.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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