diff --git a/crates/lottie/src/export.rs b/crates/lottie/src/export.rs
index eeb6e2737..88dcf4ed4 100644
--- a/crates/lottie/src/export.rs
+++ b/crates/lottie/src/export.rs
@@ -145,16 +145,18 @@ fn hex(c: [f32; 3]) -> String {
     format!("#{:02x}{:02x}{:02x}", b(c[0]), b(c[1]), b(c[2]))
 }
 
-fn mime_of(path: &str) -> &'static str {
-    let lower = path.to_ascii_lowercase();
-    if lower.ends_with(".jpg") || lower.ends_with(".jpeg") {
-        "image/jpeg"
-    } else if lower.ends_with(".webp") {
-        "image/webp"
-    } else if lower.ends_with(".gif") {
-        "image/gif"
+/// The image type `bytes` hold, by their signature; `None` for anything else.
+pub(crate) fn image_mime(bytes: &[u8]) -> Option<&'static str> {
+    if bytes.starts_with(b"\x89PNG\r\n\x1a\n") {
+        Some("image/png")
+    } else if bytes.starts_with(&[0xff, 0xd8, 0xff]) {
+        Some("image/jpeg")
+    } else if bytes.starts_with(b"GIF87a") || bytes.starts_with(b"GIF89a") {
+        Some("image/gif")
+    } else if bytes.starts_with(b"RIFF") && bytes.get(8..12) == Some(b"WEBP") {
+        Some("image/webp")
     } else {
-        "image/png"
+        None
     }
 }
 
@@ -286,7 +288,13 @@ impl Doc<'_> {
             self.ex.warn(format!("{}: cannot read image `{}` (layer exported as a null)", l.name, f.path));
             return None;
         };
-        let uri = format!("data:{};base64,{}", mime_of(&f.path), crate::base64::encode(&bytes));
+        // What a project calls a still is embedded only when it is one: the path can name any
+        // file (an imported animation's asset), and the export would carry its bytes verbatim.
+        let Some(mime) = image_mime(&bytes) else {
+            self.ex.warn(format!("{}: `{}` is not a PNG, JPEG, GIF or WebP image (layer exported as a null)", l.name, f.path));
+            return None;
+        };
+        let uri = format!("data:{mime};base64,{}", crate::base64::encode(&bytes));
         self.assets.push(json!({"id": id, "nm": it.name, "w": f.width, "h": f.height, "u": "", "p": uri, "e": 1}));
         self.done.insert(id.clone());
         Some(id)
diff --git a/crates/lottie/src/tests.rs b/crates/lottie/src/tests.rs
index 414c875f0..b941b4f61 100644
--- a/crates/lottie/src/tests.rs
+++ b/crates/lottie/src/tests.rs
@@ -725,7 +725,7 @@ fn effects_expressions_and_warnings() {
 #[test]
 fn images_embed_as_base64_and_come_back() {
     let (mut p, cid) = setup();
-    let png: Vec<u8> = vec![0x89, b'P', b'N', b'G', 1, 2, 3, 4, 5];
+    let png: Vec<u8> = vec![0x89, b'P', b'N', b'G', 0x0d, 0x0a, 0x1a, 0x0a, 1, 2, 3, 4, 5];
     let fid = p.add_item(
         "pic.png",
         Label::Lavender,
@@ -775,6 +775,17 @@ fn images_embed_as_base64_and_come_back() {
     assert!(f.path.starts_with("/tmp/image_") && f.path.ends_with(".png"), "{}", f.path);
 }
 
+#[test]
+fn image_signatures_are_matched_in_full() {
+    use crate::export::image_mime;
+    assert_eq!(image_mime(b"\x89PNG\r\n\x1a\n\0"), Some("image/png"));
+    assert_eq!(image_mime(b"GIF89a\0"), Some("image/gif"));
+    assert_eq!(image_mime(b"GIF87a\0"), Some("image/gif"));
+    // Near matches (a prefix of the signature, or text that starts with it) are not images.
+    assert_eq!(image_mime(b"\x89PNG\x01\x02\x03\x04"), None);
+    assert_eq!(image_mime(b"GIF8 is a text file"), None);
+}
+
 #[test]
 fn dotlottie_archive_roundtrip() {
     let (mut p, cid) = setup();
@@ -985,3 +996,22 @@ fn per_corner_roundness_exports_equal_radii_and_warns_when_mixed() {
     assert_eq!(rc(&json)["r"]["k"], json!(5.0));
     assert!(!warnings.iter().any(|w| w.contains("per-corner")), "{warnings:?}");
 }
+
+/// A footage item called a still whose file is not an image is not embedded.
+#[test]
+fn only_image_bytes_are_embedded() {
+    let (mut p, cid) = setup();
+    let fid = p.add_item(
+        "secret.png",
+        Label::Lavender,
+        None,
+        ItemKind::Footage(Footage { path: "/virtual/secret.png".into(), kind: FootageKind::Still, width: 8, height: 8, has_video: true, ..Default::default() }),
+    );
+    let c = comp(&p, cid);
+    let l = build::layer(&mut p, &c, "secret.png", LayerSource::Footage { item: fid }, (8, 8), None);
+    push(&mut p, cid, l);
+    let read = |path: &str| (path == "/virtual/secret.png").then(|| b"root:x:0:0:root:/root:/bin/sh\n".to_vec());
+    let res = export_comp(&p, cid, &ExportOptions::default(), &read).unwrap();
+    assert!(res.json["assets"].as_array().is_none_or(|a| a.is_empty()), "{}", res.json["assets"]);
+    assert!(res.warnings.iter().any(|w| w.contains("not a PNG")), "{:?}", res.warnings);
+}