CVE-2026-54256

### Impact The backend `FileUpload` form widget trusted an attacker-controlled `file_id` POST parameter when resolving the attachment it operates on. The lookup (`FileUpload::getFileRecord()`) resolved the posted id against the global `system_files` table without verifying that the file belonged to the widget's own relation, parent record, or deferred-binding session. Any authenticated backend user who can reach a form containing a `fileupload` field — including the built-in **My Account** avatar field, which requires no specific backend permission — could therefore target a `System\Models\File` record belonging to another user or record and: - modify its `title` and `description` via `onSaveAttachmentConfig`, and - change its `sort_order` via `onSortAttachments` (which passed posted ids straight to `setSortableOrder()`, an unscoped `UPDATE ... WHERE id = ?`). The same unscoped lookup is reached by `onLoadAttachmentConfig`, `onSaveAttachmentConfig`, and `onRemoveAttachment`. Because all attachments share the single `System\Models\File` model and `system_files` table, an attacker was not limited to other users' avatars — any attachment on any model could be referenced by id. Attachment ids are sequential integers and are easily enumerated. The confirmed impact is unauthorized integrity modification of arbitrary attachment metadata and ordering. CSRF tokens are still verified on all POST requests, so an attacker must be authenticated to the backend with a valid session. To exploit this issue an attacker needs a backend account with any level of access. ### Patches The `FileUpload` widget now scopes every `file_id` lookup to the widget's own relation, including any files bound through the current deferred-binding session, so a posted id can no longer reference an unrelated `System\Models\File` record: - `getFileRecord()` resolves the id through `getRelationObject()->withDeferred($this->sessionKey)->find(...)` rather than the global file model. This covers `o