Force lesson materials to download as attachment with readable filename

Direct CDN links opened PDFs inline (no Content-Disposition), which reads
as 'can't download' behind popup blockers / on mobile. New proxy route
streams the file with attachment + human name; lesson link points to it.

src/app/(student)/courses/[slug]/lessons/[lessonId]/page.tsx

@@ -149,9 +149,8 @@ export default async function LessonPage({ params }: Props) {
             {lesson.files.map((file) => (
               <a
                 key={file.id}
-                href={file.url}
-                target="_blank"
-                rel="noopener noreferrer"
+                href={`/api/lesson-files/${file.id}/download`}
+                download={file.name}
                 className="flex items-center gap-3 px-4 py-3 text-sm transition-colors hover:[border-color:var(--foreground)]"
                 style={{ border: "2px solid var(--border)" }}
               >

src/app/api/lesson-files/[id]/download/route.ts

@@ -0,0 +1,92 @@
+import { NextRequest, NextResponse } from "next/server";
+import { headers } from "next/headers";
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
+
+// Streams a lesson material from the CDN with a forced attachment header and a
+// human-readable filename. Direct CDN links open the PDF inline in a new tab
+// (no Content-Disposition), which reads as "can't download" for users with a
+// popup blocker or on mobile. This route makes the browser save the file.
+
+const CONTENT_TYPES: Record<string, string> = {
+  pdf: "application/pdf",
+  zip: "application/zip",
+  doc: "application/msword",
+  docx: "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+  ppt: "application/vnd.ms-powerpoint",
+  pptx: "application/vnd.openxmlformats-officedocument.presentationml.presentation",
+  xls: "application/vnd.ms-excel",
+  xlsx: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+  png: "image/png",
+  jpg: "image/jpeg",
+  jpeg: "image/jpeg",
+  webp: "image/webp",
+};
+
+function extFromUrl(url: string): string {
+  const clean = url.split("?")[0];
+  const dot = clean.lastIndexOf(".");
+  return dot >= 0 ? clean.slice(dot + 1).toLowerCase() : "";
+}
+
+export async function GET(
+  _req: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session) {
+    return NextResponse.json({ error: "unauthorized" }, { status: 401 });
+  }
+
+  const { id } = await params;
+  const file = await prisma.lessonFile.findUnique({
+    where: { id },
+    include: { lesson: { include: { module: { select: { courseId: true } } } } },
+  });
+  if (!file) {
+    return NextResponse.json({ error: "not_found" }, { status: 404 });
+  }
+
+  // Access: staff always; students must be enrolled in the file's course.
+  const role = session.user.role;
+  if (role !== "admin" && role !== "curator") {
+    const enrollment = await prisma.courseEnrollment.findFirst({
+      where: { userId: session.user.id, courseId: file.lesson.module.courseId },
+      select: { userId: true },
+    });
+    if (!enrollment) {
+      return NextResponse.json({ error: "forbidden" }, { status: 403 });
+    }
+  }
+
+  const upstream = await fetch(file.url);
+  if (!upstream.ok || !upstream.body) {
+    return NextResponse.json({ error: "file_unavailable" }, { status: 502 });
+  }
+
+  const ext = extFromUrl(file.url);
+  const contentType =
+    upstream.headers.get("content-type") ??
+    CONTENT_TYPES[ext] ??
+    "application/octet-stream";
+
+  // Build a readable filename: "<name>.<ext>" (avoid doubling the extension).
+  let downloadName = file.name.trim();
+  if (ext && !downloadName.toLowerCase().endsWith(`.${ext}`)) {
+    downloadName += `.${ext}`;
+  }
+  const asciiName = downloadName.replace(/[^\x20-\x7E]+/g, "_").replace(/"/g, "");
+  const disposition =
+    `attachment; filename="${asciiName}"; ` +
+    `filename*=UTF-8''${encodeURIComponent(downloadName)}`;
+
+  const respHeaders: Record<string, string> = {
+    "Content-Type": contentType,
+    "Content-Disposition": disposition,
+    "Cache-Control": "private, no-store",
+  };
+  const len = upstream.headers.get("content-length");
+  if (len) respHeaders["Content-Length"] = len;
+
+  return new NextResponse(upstream.body, { status: 200, headers: respHeaders });
+}