Add screenshot upload and change form method to POST.

index.html

@@ -29,7 +29,7 @@
     <p>This is a prototype for FediBlock, a central blocklist for the Fediverse.</p>
 
     <h2>Form</h2>
-    <form action="/add" method="get">
+    <form method="post" action="/add" enctype="multipart/form-data">
       <label for="instance">Instance:</label>
       <input type="text" id="instance" name="instance" placeholder="example.com" autofocus required><br>
 
@@ -41,6 +41,9 @@
       <input type="url" id="receipts" name="receipts" placeholder="https://example.com/@user/1234567890" required>
       <input type="button" id="add_receipt" value="➕" disabled><br>
 
+      <label for="screenshot">Screenshot:</label>
+      <input type="file" id="screenshot" name="screenshot"><span>(optional)</span><br>
+
       <label for="description">Description:</label><br>
       <textarea id="description" name="description" cols="80" rows="20" required></textarea><br>