The attachment bar reused an <img> element whose CSS `display: block` rule
overrode the UA stylesheet `[hidden] { display: none }`, so the empty image
stayed visible as a broken "Attached image preview" thumbnail next to the
audio player. Re-assert [hidden] with matching specificity for both the
image and audio elements, and prefix audio filenames with a music-note
glyph so the attachment reads as audio at a glance.