[fork-CI] [20.0][MIG] attachment_preview: Migration to 20.0

[dnplkndll]

Port of attachment_preview from 19.0 to 20.0. Follows the OCA migration guide.

No dependency pin: it depends only on core web and mail.

Carries a forward-port of #14 as its own [FIX] commit — that fix is still unmerged on 19.0, and 20.0 needs it for the same reason 19.0 did.

Non-mechanical adaptations worth flagging

The Python side is two API removals. The JavaScript is most of the work, because 20.0 removed both things this module was built on.

• jQuery is gone from 20.0 entirely — no lib, no bundle entry, and @web/core/ensure_jquery deleted (zero core users). All nine call sites are vanilla DOM now. Where jQuery silently absorbed a null — an unrendered t-ref, a selector matching nothing — the replacement guards explicitly instead of throwing.
• Owl 3 removed the static Component.env (zero core files use it). The widget reads this.env.bus; showPreview takes an env argument, since it is a free function and both callers are component methods.
• Owl 3 raises on static props, and the throw propagates out of whatever mounted the component. With none to declare, core's own answer is to omit it.
• Owl 3 resolves a bare template identifier against the render context, not the component, so the four t-on-click handlers are qualified with this. — unqualified they are silently undefined.
• The Store hook was renamed and reshaped: _to_store_defaults → _store_attachment_fields(res: Store.FieldList, **kwargs), which appends to the list it is handed instead of returning one. The old name could not be reached even if it still existed, because Store.add only accepts a method name starting with _store_ and ending with _fields. Left unported, the extension never reaches the client and every preview button disappears — with nothing failing anywhere.
• ir.attachment.datas is gone (raw now), and Binary fields are handed over as a BinaryValue holding RAW bytes where 19.0 gave base64. So magic.from_buffer needs real bytes, and the data-URL fallback must b64-encode rather than decode. bin_size no longer changes the value either.
• The preview button attaches to 20.0's download button, now oi btn btn-link o_download_file_button with data-icon="download" — the old button.fa-download selector matched nothing. Keyed on the semantic class, scoped to the component's own DOM, and declining to attach when the match is ambiguous rather than decorating another field's button.
• FontAwesome is gone, so the six template icons are oi + a data-icon ligature.
• Owl 3 replaced the reactivity primitives outright. useState and useRef are not exported at all, and core calls neither: state is proxy({...}), and a ref is a signal typed t.ref(), declared as a class field, bound with t-ref="this.<name>" and read by calling it. Until this was fixed the widget threw useState is not a function from setup() and took the whole form view down behind an "Oops!" dialog, on any record carrying an attachment.
• AttachmentList.previewableAttachments is gone. Core builds the equivalent inline in its own onClickAttachment, so the override takes it from props.attachmentGroups the same way. Left alone the list arrived undefined, showPreview fell through to its window.open branch, and the side panel silently never opened — with no error anywhere.
• The click handler is handed its event rather than reading the implicit global window.event, which is only set while a native handler is on the stack. Owl 3 calls it through a wrapper, so the global was undefined and the click did nothing, again silently.

3/3 Python tests pass. Note what that does not cover: there are no JS tests and no tours, so the whole list above is verified by a browser walk on a migrated production copy rather than by CI — opening a record with a real PDF, asserting the preview button renders (which also proves extension reached the client), clicking it, and checking the panel opens with the iframe pointed at the viewer and zero client errors. Four of the items above were found only that way, three of them failing silently.

AI-assisted (Claude Code); every change reviewed, tested, and owned by the author.