rename .html templates to .jinja2 for more consistency

templates/upload.jinja2

@@ -0,0 +1,21 @@
+{% extends "base.html" %}
+
+{% block content %}
+<h1>Upload new file</h1>
+
+<p>Uploads are no more supported. Please use this syntax instead for external images: <code>![alt text](https://www.example.com/path/to/image.jpg)</code>.</p>
+
+<form method="POST" enctype="multipart/form-data">
+  <div>
+    <label for="name">Name: </label>
+    <input type="text" id="name-input" name="name" required maxlength="256" disabled="">
+  </div>
+  <div>
+    <label for="file">File: </label>
+    <input type="file" id="file-input" name="file" accept="image/jpeg, image/png" disabled="">
+  </div>
+  <div>
+    <input type="submit" value="Upload" disabled="">
+  </div>
+</form>
+{% endblock %}