[17.0][ADD] attachment_preview_xml: preview XML attachments as a collapsible tree

[alan196]

Summary

New module that renders XML attachments in the standard file viewer as an indented, collapsible tree instead of a wall of raw text.

Context

ir.attachment._check_contents neutralizes xml-like mimetypes to text/plain for every user without write access on views, so XML attachments end up in the plain-text branch of the FileViewer. Machine-generated XML (electronic invoices such as CFDI or UBL, bank statements, EDI messages) is usually serialized on a single line, which makes that preview unreadable.

How it works

• A JS patch on the mail Attachment model adds isXml (by extension, or by mimetype when it survives), removes those files from the isText branch and keeps them viewable.
• XmlViewer fetches the attachment, parses it with DOMParser and renders the tree with plain <details>/<summary> elements, so collapsing is native HTML: no library, no server-side conversion, no external service.
• Malformed XML, or content over 5 MB, falls back to the raw text instead of showing nothing.
• No Python code and no new dependencies: the module ships assets only.

Test plan

• QUnit tests on the parser (hierarchy, attributes, comments, text-only nodes, malformed input), run in CI through an HttpCase that loads the module suite in a real browser.
• Manually validated on 17.0 with a single-line CFDI 4.0 attached to a chatter: the tree renders and every branch collapses; a deliberately malformed XML falls back to raw text.

[pedrobaeza]

Tested on runboat