security: whitelist allowed image MIME types for uploads

Only accept common image formats (JPEG, PNG, GIF, WebP, AVIF, SVG)
on the upload endpoint. Non-image uploads now return HTTP 415 with a
structured JSON error listing the allowed types. Includes a test.

test/admin_test.rb

@@ -124,6 +124,19 @@ class AdminTest < Minitest::Test
     assert_equal 403, last_response.status
   end
 
+  def test_upload_rejects_unsupported_media_type
+    login
+    get "/admin/posts/new"
+    token = csrf(last_response.body)
+    txt_path = File.join(@dir, "readme.txt")
+    File.binwrite(txt_path, "hello")
+    post "/admin/uploads",
+         "_csrf" => token,
+         "file" => Rack::Test::UploadedFile.new(txt_path, "text/plain")
+    assert_equal 415, last_response.status
+    assert_equal "unsupported_media_type", JSON.parse(last_response.body)["error"]
+  end
+
   def test_upload_rejects_file_too_large
     login
     get "/admin/posts/new"