Add docs on our custom web elements

docs/api/file-object.md

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+# `File` object
+
+The DOM's File interface provides abstraction around native files, in order to
+let users work on native files directly with HTML5 file API, atom-shell has
+added a `path` attribute to `File` interface which exposes the file's real path
+on filesystem.
+
+Example on getting real path of a dragged file:
+
+```html
+<div id="holder">
+  Drag your file here
+</div>
+
+<script>
+  var holder = document.getElementById('holder');
+  holder.ondragover = function () {
+    return false;
+  };
+  holder.ondragleave = holder.ondragend = function () {
+    return false;
+  };
+  holder.ondrop = function (e) {
+    e.preventDefault();
+    var file = e.dataTransfer.files[0];
+    console.log('File you dragged here is', file.path);
+    return false;
+  };
+</script>
+```