#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/history.html

@@ -11,7 +11,7 @@
       <colgroup><col class="col-a"><col class="col-b"><col class="col-c"><col class="col-d"><col class="col-e"><col class="col-f"><col class="col-g"></colgroup>
       <thead><tr>
         <th data-sort="title" class="sortable">Paste<span class="sort-ind"></span></th>
-        <th data-sort="language" class="sortable">Language<span class="sort-ind"></span></th>
+        <th data-sort="type" class="sortable">Type<span class="sort-ind"></span></th>
         <th data-sort="size" class="sortable">Size<span class="sort-ind"></span></th>
         <th data-sort="view_count" class="sortable">Views<span class="sort-ind"></span></th>
         <th data-sort="created_at" class="sortable">Created<span class="sort-ind"></span></th>