diff --git a/app/api/staff/file/route.ts b/app/api/staff/file/route.ts
index 6864a52..b2971ca 100644
--- a/app/api/staff/file/route.ts
+++ b/app/api/staff/file/route.ts
@@ -31,6 +31,28 @@ import { resolveMime } from "@/lib/mime.mjs";
 
 const MAX_BYTES = 10 * 1024 * 1024;
 
+// Only mimes the lightbox actually renders inline are allowed through the
+// `Content-Disposition: inline` path. Everything else gets coerced to
+// application/octet-stream + attachment so it always downloads.
+//
+// Why this matters: our /api/upload route validates uploaded mimes
+// against an allowlist, but the underlying civicrm_file row can be
+// populated by other paths too — a Civi admin uploading directly through
+// the CiviCRM UI, a future Civi import, etc. If any of those routes
+// stored mime_type="text/html" or "image/svg+xml", an inline serve from
+// this same-origin proxy would let arbitrary script run against
+// /api/* and the staff key. Defence in depth: don't trust mime_type
+// when the response carries `inline`.
+//
+// Explicitly NOT in this set: svg (script-bearing), html, xml, any text/*.
+const SAFE_INLINE_MIMES = new Set<string>([
+  "image/png",
+  "image/jpeg",
+  "image/gif",
+  "image/webp",
+  "application/pdf",
+]);
+
 function isCiviStubMode(): boolean {
   return !(
     process.env.CIVI_BASE_URL &&
@@ -154,17 +176,28 @@ export async function GET(req: NextRequest) {
     return NextResponse.json({ error: "File too large for inline preview." }, { status: 413 });
   }
 
-  const mime = resolveMime(
+  const resolved = resolveMime(
     row.mime_type ?? upstreamRes.headers.get("content-type"),
     row.name,
   );
+  // Inline is only allowed for mimes the lightbox actually renders. Anything
+  // else (incl. an attacker-controlled mime_type from a non-allowlisted
+  // upload path) is downgraded to octet-stream + attachment so the browser
+  // downloads instead of executing.
+  const inlineSafe = !wantsDownload && SAFE_INLINE_MIMES.has(resolved);
+  const mime = inlineSafe ? resolved : "application/octet-stream";
+  const disposition = inlineSafe ? "inline" : "attachment";
   const safeName = (row.name ?? `file-${fileId}`).replace(/[\r\n"\\]/g, "_");
 
   const headers: Record<string, string> = {
     "Content-Type": mime,
-    "Content-Disposition": `${wantsDownload ? "attachment" : "inline"}; filename="${safeName}"`,
+    "Content-Disposition": `${disposition}; filename="${safeName}"`,
     "Cache-Control": "private, no-store",
     "X-Content-Type-Options": "nosniff",
+    // Belt-and-suspenders: even if a future change accidentally lets a
+    // scriptable mime through the allowlist, the sandbox CSP keeps the
+    // response from running script or talking to anything else.
+    "Content-Security-Policy": "sandbox; default-src 'none'; img-src 'self'; object-src 'self'",
   };
   if (Number.isFinite(contentLength)) {
     headers["Content-Length"] = String(contentLength);