Do not offer a player for a file that is not audio or video

The media-type filter stopped new image enclosures being fetched, but ones
already on disk still got a play button and an <audio> element, because
the UI tested for a path rather than for a playable type. Four places did
this, including play() itself, which picked the first downloaded enclosure
whatever it was.

isPlayable() checks audio/* or video/*, falling back to the extension when
a feed declares no type. A downloaded non-media file now shows as its kind
with Save and Delete, so it stays available without posing as an episode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AdXho5tTkjFLeUXKbEjKBh

tests/ui/global-setup.js

@@ -34,6 +34,12 @@ token = "${TOKEN}"
 url = "http://127.0.0.1:8792/show.xml"
 auto_download = true
 
+# Downloads its image, so the UI has a file that is not playable to deal with.
+[feeds.picture-blog]
+url = "http://127.0.0.1:8792/pics.xml"
+auto_download = true
+media_types = ["image"]
+
 [feeds.test-subscriptions]
 url = "http://127.0.0.1:8792/subs.opml"
 auto_download = false