v3: code quality pass, various bug fixes

api/utils/upload.ts

@@ -1,4 +1,5 @@
 import type { Context } from "@oak/oak";
+import { APIErrorCode, APIException } from "../model/interfaces.ts";
 
 export const UPLOADS_DIR = "api/uploads";
 export const DUMPS_DIR = `${UPLOADS_DIR}/dumps`;
@@ -35,6 +36,26 @@ export function detectImageMime(data: Uint8Array): string | null {
   return null;
 }
 
+/** Validates image upload data: checks size and MIME. Returns the detected MIME type or throws APIException. */
+export function validateImageUpload(data: Uint8Array): string {
+  if (data.length > MAX_IMAGE_SIZE) {
+    throw new APIException(
+      APIErrorCode.BAD_REQUEST,
+      400,
+      "File too large (max 5 MB)",
+    );
+  }
+  const mime = detectImageMime(data);
+  if (!mime) {
+    throw new APIException(
+      APIErrorCode.BAD_REQUEST,
+      400,
+      "File content is not a recognised image (JPEG, PNG, GIF, WebP)",
+    );
+  }
+  return mime;
+}
+
 export async function serveUploadedFile(
   ctx: Context,
   filePath: string,