feat: accept more plain-text file uploads (#1242)

Allow .typ, .patch, .diff, .gitpatch, and .gitignore attachments
so users do not have to rename those files to .txt first.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Backport shared file recognition and picker accept configuration to release-1.23.

(cherry picked from commit acb241e2772247a4573451f3daf303b08f56b69d)

(cherry picked from pro commit ee17bd609e513f5c95393bbb8bbf7303b72209e7)