feat(uploads): restrict to webp/avif with conversion hint

lib/volumen/server.rb

@@ -19,9 +19,7 @@ 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
+    ALLOWED_UPLOAD_TYPES = %w[image/webp image/avif].freeze
     SLUG_REGEX = /\A[a-z0-9](?:[a-z0-9._-]*[a-z0-9])?\z/
 
     register ApiRoutes
@@ -248,7 +246,11 @@ module Volumen
       content_type = upload[:type].to_s.split(";").first.to_s.strip.downcase
       return if ALLOWED_UPLOAD_TYPES.include?(content_type)
 
-      halt(415, json("error" => "unsupported_media_type", "allowed" => ALLOWED_UPLOAD_TYPES))
+      halt(415, json("error" => "unsupported_format",
+                     "message" => "Only WebP (.webp) and AVIF (.avif) images are supported. " \
+                                  "Please convert your image before uploading — for example with " \
+                                  "`cwebp` (JPEG/PNG → WebP) or `avifenc` (PNG/JPEG → AVIF).",
+                     "allowed" => ALLOWED_UPLOAD_TYPES))
     end
 
     def check_login_rate_limit!

lib/volumen/web/views/form.erb

@@ -299,13 +299,25 @@
 
     // --- Image upload ------------------------------------------------------
 
-    function uploadImage(file, onDone) {
+    function uploadImage(file, onSuccess, onError) {
       var data = new FormData();
       data.append("file", file);
       data.append("_csrf", csrf);
       fetch(uploadUrl, { method: "POST", body: data })
-        .then(function (response) { return response.json(); })
-        .then(function (json) { if (json.url) { onDone(json.url); } });
+        .then(function (response) {
+          return response.json().then(function (json) { return { ok: response.ok, json: json }; });
+        })
+        .then(function (result) {
+          if (result.ok && result.json.url) {
+            onSuccess(result.json.url);
+          } else {
+            onError(result.json.message || ("Upload failed (HTTP " + (result.json.error || "error") + ")"));
+          }
+        });
+    }
+
+    function showUploadError(message) {
+      alert("Upload failed:\n\n" + message);
     }
 
     imageInput.addEventListener("change", function () {
@@ -317,7 +329,7 @@
         } else {
           exec("insertImage", url);
         }
-      });
+      }, showUploadError);
       imageInput.value = "";
     });
 
@@ -346,7 +358,7 @@
       uploadImage(file, function (url) {
         coverInput.value = url;
         refreshCoverPreview();
-      });
+      }, showUploadError);
       coverFile.value = "";
     });
     coverInput.addEventListener("input", refreshCoverPreview);