[ticket/14771] Support playing audio attachments

Add Audio attachments category, Audio attachments group.
add most audio supported extensions to the audio group.
presserve audio extensions MIMEtypes.
modify the attachment style file to view the audio attachment in
AUDIO HTML5 tag.

PHPBB3-14771

phpBB/includes/acp/acp_attachments.php

@@ -1410,6 +1410,7 @@ class acp_attachments
 		$types = array(
 			ATTACHMENT_CATEGORY_NONE		=> $user->lang['NO_FILE_CAT'],
 			ATTACHMENT_CATEGORY_IMAGE		=> $user->lang['CAT_IMAGES'],
+			ATTACHMENT_CATEGORY_AUDIO		=> $user->lang['CAT_AUDIO_FILES'],
 		);
 
 		if ($group_id)