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.

lib/volumen/admin_routes.rb

@@ -92,6 +92,12 @@ module Volumen
           halt(413, json("error" => "file_too_large", "max_bytes" => Server::MAX_UPLOAD_BYTES))
         end
 
+        content_type_str = upload[:type].to_s.split(";").first.to_s.strip.downcase
+        unless Server::ALLOWED_UPLOAD_TYPES.include?(content_type_str)
+          halt(415, json("error" => "unsupported_media_type",
+                        "allowed" => Server::ALLOWED_UPLOAD_TYPES))
+        end
+
         json("url" => store.store_upload(upload[:filename], upload[:tempfile]))
       end
 

lib/volumen/server.rb

@@ -18,6 +18,9 @@ module Volumen
     MAX_LOGIN_ATTEMPTS = 10
     LOGIN_WINDOW = 60 # seconds
     MAX_UPLOAD_BYTES = 10 * 1024 * 1024 # 10 MB
+    ALLOWED_UPLOAD_TYPES = %w[
+      image/jpeg image/png image/gif image/webp image/avif image/svg+xml
+    ].freeze
     SLUG_REGEX = /\A[a-z0-9](?:[a-z0-9._-]*[a-z0-9])?\z/
 
     register ApiRoutes