diff --git a/src/app/(frontend)/intelligence/missions/actions.ts b/src/app/(frontend)/intelligence/missions/actions.ts
index 07f31ae..4706cee 100644
--- a/src/app/(frontend)/intelligence/missions/actions.ts
+++ b/src/app/(frontend)/intelligence/missions/actions.ts
@@ -205,6 +205,15 @@ export async function uploadSideOpModlist(
     if (file.size > 10 * 1024 * 1024) {
       return { success: false, error: "Mod list file must be 10 MB or smaller." };
     }
+    // Only HTML mod lists exported from the Arma 3 launcher are supported. Some
+    // browsers report an empty file.type, so also accept by extension.
+    const isHtml = file.type === "text/html" || /\.html?$/i.test(file.name);
+    if (!isHtml) {
+      return {
+        success: false,
+        error: "Mod list must be an HTML file exported from the Arma 3 launcher.",
+      };
+    }
 
     const buffer = Buffer.from(await file.arrayBuffer());
     const created = await payload.create({