#235: rename Language column to Type; attachment pastes show file extension

- history/mine table column 'Language' -> 'Type' (data-sort key 'type')
- /api/public and /api/mine rows gain a 'type' field: file extension for
  attachment pastes, stored language otherwise (default 'text')
- paste page stats label 'Language' -> 'Type'; summary line uses the same
  label; get-paste JSON gains 'type'
- store list queries LEFT JOIN attachments to expose the filename
- table.js sorting accepts the 'type' key

internal/web/templates/paste.html

@@ -20,7 +20,7 @@
       </button>
       <div class="stats-body" id="stats-body" hidden>
         <div class="stats-grid">
-          <span class="stats-k">Language</span><span class="stats-v">{{if .Language}}{{.Language}}{{else}}text{{end}}</span>
+          <span class="stats-k">Type</span><span class="stats-v">{{.TypeLabel}}</span>
           <span class="stats-k">Size</span><span class="stats-v">{{.SizeHuman}} ({{.LineCount}} lines)</span>
           <span class="stats-k">Views</span><span class="stats-v">{{.ViewCount}}</span>
           <span class="stats-k">Created</span><span class="stats-v" data-ts="{{.CreatedAtUnix}}">{{.CreatedAgo}}</span>