Feature: preview composer attachments inline (click to open)

[dealerweb]

Summary

The message viewer already lets you click an attachment to preview it, but the composer/draft view only offered download/remove. This adds click-to-preview parity: clicking an attachment chip in the composer opens the same FilePreviewModal.

Changes

• The attachment chip's name/size becomes a button when the attachment has content (a local File, or - for forwarded attachments - an uploaded blob) and the type is previewable (isFilePreviewable); it is never clickable mid-upload. The remove (X) button is unchanged.
• getFileContent prefers the in-memory File (no network round-trip) and falls back to composerClient.fetchBlob for forwarded attachments (blobId only).
• Reuses the existing FilePreviewModal, so all rendering (image / PDF / pdf.js-mobile / video / audio / text / html) and the open-in-new-tab inline-safe-MIME gate come for free.

Type of Change

• New feature (non-breaking change that adds functionality)

Checklist

• Contributing Guide read
• Code follows project style and conventions
• npm run typecheck && npm run lint passes
• Build passes (npm run build)
• Changes tested locally
• Translations updated (locales/) - n/a, no new strings (the tooltip reuses the filename)

Notes for Reviewers

• No new dependencies and no new locale keys.
• Security: adds no new egress. Preview rendering and the open-in-new-tab gate are the modal's (html renders in <iframe sandbox="">; the open-in-new-tab button is gated by isMimeTypeSafeForInlinePreview). The local-file download path uses an <a download> (forces a save, never executes), so a script-bearing attachment can't run in-origin.
• next lint errors on Next 16 repo-wide (unrelated); ESLint on the changed file passes (one pre-existing exhaustive-deps warning, not from this change).
• Tested on a live Stalwart deployment: dragging files into a new mail / draft, clicking the chip opens the preview; remove still works.