Restrict the "Choose file…" document picker to font MIME types
(font/ttf, font/otf, font/sfnt, font/collection plus legacy
application/* font aliases) instead of */*, so only fonts are
selectable. CustomFontStore's Font.Builder validation still guards
anything a provider mislabels.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>