Web: media preview panel shows "0 B" for uploaded image attachments

Bug

In the web UI, clicking an image attachment in a user message opens the preview panel with 0 B as the file size, and the fallback title is "ReadMediaFile image" even though the media is a user upload, not a tool result.

Root cause

ChatPane.userAttachmentMedia() builds the ToolMedia for the preview with no bytes field (a TurnAttachment for image/video carries no size). In v2 sessions the image is rehydrated from the agent blob store as a bare data: URL with no size metadata either, so openMediaPreview falls back to size: 0 (useFilePreview.ts) and the header renders "0 B". The same fallback produces the misleading "ReadMediaFile image" label when the media has no path.

Expected

The preview header shows the real byte size, and a generic Image/Video label for unnamed uploads.

Where

• apps/kimi-web/src/components/chat/ChatPane.vue (userAttachmentMedia)
• apps/kimi-web/src/composables/useFilePreview.ts (openMediaPreview)

Read userAttachmentMedia in apps/kimi-web/src/components/chat/ChatPane.vue and openMediaPreview in apps/kimi-web/src/composables/useFilePreview.ts, then trace the image and video attachment data for both regular and v2 sessions. Verify the preview header reports the uploaded byte size and uses generic Image/Video labels when no path is available.