diff --git a/crates/engine/src/cmd/file.rs b/crates/engine/src/cmd/file.rs
index 08bdaf0ce3..6494aa55d4 100644
--- a/crates/engine/src/cmd/file.rs
+++ b/crates/engine/src/cmd/file.rs
@@ -384,6 +384,9 @@ fn export_png(s: &mut Session, v: &Value) -> CmdResult {
     // the on-screen cue while editing the body, not part of the page.
     let mut opts = wordcraft_render::RenderOptions::default();
     opts.display.dim_header = false;
+    if !path.to_ascii_lowercase().ends_with(".png") {
+        return Err(CmdError::Params("path must end in .png".into()));
+    }
     let img = wordcraft_render::render_page(&s.doc, pg, scale, &opts);
     let png = img.to_png();
     crate::io::write_file(path, &png).map_err(CmdError::Failed)?;
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 8c777dba6d..305e640d7e 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -9342,3 +9342,27 @@ fn headless_ui_requests_stay_bounded() {
     let size: usize = s.ui_requests.iter().map(|r| r.to_string().len()).sum();
     assert!(size < 4 << 20, "queued {size} bytes");
 }
+
+/// `file.exportPng` writes only to `.png` paths, so automation can't replace other files with
+/// page images (#3696).
+#[cfg(not(target_arch = "wasm32"))]
+#[test]
+fn export_png_refuses_a_path_that_is_not_png() {
+    let dir = scratch_dir("export-png-extension");
+    let mut s = s();
+    run(&mut s, "text.insert", json!({"text": "Hello"}));
+    for name in ["notes.txt", "config", "report.docx", "page.png.txt"] {
+        let target = dir.join(name);
+        std::fs::write(&target, b"precious user notes").unwrap();
+        let r = s.run("file.exportPng", &json!({"path": target.to_string_lossy()}));
+        assert!(matches!(r, Err(crate::CmdError::Params(_))), "{name}: {r:?}");
+        assert_eq!(std::fs::read(&target).unwrap(), b"precious user notes", "{name} was changed");
+    }
+    // A .png path still works, whatever its case.
+    for name in ["page.png", "PAGE.PNG"] {
+        let target = dir.join(name);
+        s.run("file.exportPng", &json!({"path": target.to_string_lossy()})).unwrap();
+        assert!(std::fs::read(&target).unwrap().starts_with(b"\x89PNG"), "{name} is not a PNG");
+    }
+    let _ = std::fs::remove_dir_all(&dir);
+}