Hook up attach file shortcut, add it to user guide. Bug 743565.

help/C/shortcuts.page

@@ -125,6 +125,10 @@
         <title>Composer shortcuts</title>
         <p>These shortcuts are active whenever focus is in a composer.</p>
         <table frame="all" rules="rows">
+            <tr>
+                <td><p>Attach file</p></td>
+                <td><p><keyseq><key>Ctrl</key><key>T</key></keyseq></p></td>
+            </tr>
             <tr>
                 <td><p>Quote text</p></td>
                 <td><p> <keyseq><key>Ctrl</key><key>]</key></keyseq> </p></td>

ui/composer_accelerators.ui

@@ -2,6 +2,8 @@
     <accelerator action="undo" />
     <accelerator action="redo" />
 
+    <accelerator action="add attachment" />
+
     <accelerator action="cut" />
     <accelerator action="copy" />
     <accelerator action="copy link" />