core: add file fallback

packages/hydrooj/src/handler/contest.ts

@@ -320,7 +320,12 @@ export class ContestProblemFileDownloadHandler extends ContestProblemHandler {
         if (typeof this.pdoc.docId === 'string') this.pdoc.docId = this.pdoc.docId.split(':')[1];
         const target = `problem/${this.pdoc.domainId}/${this.pdoc.docId}/additional_file/${filename}`;
         const file = await storage.getMeta(target);
-        if (!file) throw new NotFoundError(filename);
+        if (!file) {
+            this.response.redirect = await storage.signDownloadLink(
+                target, noDisposition ? undefined : filename, false, 'user',
+            );
+            return;
+        }
         this.response.etag = file.etag;
         const type = lookup(filename).toString();
         const shouldProxy = ['image', 'video', 'audio', 'pdf', 'vnd'].filter((i) => type.includes(i)).length;