address review: match full PNG and GIF signatures

crates/lottie/src/export.rs

@@ -146,12 +146,12 @@ fn hex(c: [f32; 3]) -> String {
 }
 
 /// The image type `bytes` hold, by their signature; `None` for anything else.
-fn image_mime(bytes: &[u8]) -> Option<&'static str> {
-    if bytes.starts_with(b"\x89PNG") {
+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"GIF8") {
+    } 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")

crates/lottie/src/tests.rs

@@ -672,7 +672,7 @@ 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,
@@ -722,6 +722,17 @@ 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();