DEV: Expand UploadMarkdown generation capabilities (#15930)

The chat quoting mechanism will need to be able to generate
markdown for all kinds of uploads. The UploadMarkdown class
was missing generation for video and audio uploads. This
commit adds that in, and also expands the server-side regex
recognition of FileHelper types to match those in uploads.js,
and adds a spec for UploadMarkdown

lib/upload_markdown.rb

@@ -8,6 +8,8 @@ class UploadMarkdown
   def to_markdown(display_name: nil)
     if FileHelper.is_supported_image?(@upload.original_filename)
       image_markdown
+    elsif FileHelper.is_supported_playable_media?(@upload.original_filename)
+      playable_media_markdown
     else
       attachment_markdown(display_name: display_name)
     end
@@ -23,4 +25,15 @@ class UploadMarkdown
 
     "[#{display_name}|attachment](#{@upload.short_url})#{human_filesize}"
   end
+
+  def playable_media_markdown
+    type = \
+      if FileHelper.is_supported_audio?(@upload.original_filename)
+        "audio"
+      elsif FileHelper.is_supported_video?(@upload.original_filename)
+        "video"
+      end
+    return attachment_markdown if !type
+    "![#{@upload.original_filename}|#{type}](#{@upload.short_url})"
+  end
 end