From 624463c5bab0693bcef13fd1c63c40f7fc85edbf Mon Sep 17 00:00:00 2001
From: Tyler Hillery <tyhillery@gmail.com>
Date: Wed, 23 Sep 2026 13:04:05 -0500
Subject: [PATCH] fix(s3): default CopyObject content-type to
 binary/octet-stream on REPLACE (#8073)

fix: default CopyObject content-type to binary/octet-stream on REPLACE
---
 .../e2e_test/src/copy_object_metadata_test.rs | 22 ++++++++++++++++---
 rustfs/src/app/object/copy.rs                 | 11 ++++++++--
 2 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/crates/e2e_test/src/copy_object_metadata_test.rs b/crates/e2e_test/src/copy_object_metadata_test.rs
index ac416c750..e11082417 100644
--- a/crates/e2e_test/src/copy_object_metadata_test.rs
+++ b/crates/e2e_test/src/copy_object_metadata_test.rs
@@ -298,7 +298,11 @@ mod tests {
         assert_eq!(empty_head_resp.content_disposition(), None);
         assert_eq!(empty_head_resp.content_encoding(), None);
         assert_eq!(empty_head_resp.content_language(), None);
-        assert_eq!(empty_head_resp.content_type(), None);
+        assert_eq!(
+            empty_head_resp.content_type(),
+            Some("binary/octet-stream"),
+            "REPLACE without an explicit content type must default to binary/octet-stream, matching S3"
+        );
         assert_eq!(empty_head_resp.expires_string(), None);
         assert_eq!(empty_head_resp.website_redirect_location(), None);
 
@@ -394,7 +398,15 @@ mod tests {
             assert_eq!(head.content_disposition(), (field == "content-disposition").then_some("attachment"));
             assert_eq!(head.content_encoding(), (field == "content-encoding").then_some("gzip"));
             assert_eq!(head.content_language(), (field == "content-language").then_some("de"));
-            assert_eq!(head.content_type(), (field == "content-type").then_some("text/field"));
+            assert_eq!(
+                head.content_type(),
+                Some(if field == "content-type" {
+                    "text/field"
+                } else {
+                    "binary/octet-stream"
+                }),
+                "REPLACE without an explicit content type must default to binary/octet-stream, matching S3"
+            );
             assert_eq!(
                 head.expires_string(),
                 (field == "expires").then_some(replacement_expires_http_date.as_str())
@@ -420,7 +432,11 @@ mod tests {
             .send()
             .await
             .expect("HEAD failed for metadata collision case");
-        assert_eq!(collision_head.content_type(), None);
+        assert_eq!(
+            collision_head.content_type(),
+            Some("binary/octet-stream"),
+            "user-metadata under the reserved 'content-type' key must not satisfy the system content-type default"
+        );
         assert_eq!(collision_head.content_encoding(), None);
         assert_eq!(
             collision_head.metadata().and_then(|metadata| metadata.get("content-type")),
diff --git a/rustfs/src/app/object/copy.rs b/rustfs/src/app/object/copy.rs
index 614fc1b85..4ce0774ff 100644
--- a/rustfs/src/app/object/copy.rs
+++ b/rustfs/src/app/object/copy.rs
@@ -607,9 +607,16 @@ impl DefaultObjectUsecase {
         // Handle MetadataDirective REPLACE: replace user metadata while preserving system metadata.
         // System metadata (compression, encryption) is added after this block to ensure
         // it's not cleared by the REPLACE operation.
-        if let Some(replacement_metadata) = replacement_metadata {
+        if let Some(mut replacement_metadata) = replacement_metadata {
+            // S3 defaults an object's content type to `binary/octet-stream` when none is
+            // supplied. REPLACE must apply that same default instead of leaving
+            // content-type unset.
+            if !replacement_metadata.contains_key("content-type") {
+                replacement_metadata.insert("content-type".to_string(), "binary/octet-stream".to_string());
+            }
+            let effective_content_type = replacement_metadata.get("content-type").cloned();
             user_defined = replacement_metadata;
-            src_info.content_type = content_type.clone();
+            src_info.content_type = effective_content_type;
             src_info.content_encoding = content_encoding.as_deref().and_then(normalize_content_encoding_for_storage);
             src_info.expires = expires_timestamp.map(OffsetDateTime::from);
         } else if metadata_directive.is_some() || website_redirect_location.is_some() {