s3: fix if-match error (#7277)

* s3: fix if-match error

* add more checks

* minor

* minor

---------

Co-authored-by: chrislu <chris.lu@gmail.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>

weed/s3api/s3api_object_handlers_put.go

@@ -1257,6 +1257,11 @@ func (s3a *S3ApiServer) getObjectETag(entry *filer_pb.Entry) string {
 	if etagBytes, hasETag := entry.Extended[s3_constants.ExtETagKey]; hasETag {
 		return string(etagBytes)
 	}
+	// Check for Md5 in Attributes (matches filer.ETag behavior)
+	// Note: len(nil slice) == 0 in Go, so no need for explicit nil check
+	if entry.Attributes != nil && len(entry.Attributes.Md5) > 0 {
+		return fmt.Sprintf("\"%x\"", entry.Attributes.Md5)
+	}
 	// Fallback: calculate ETag from chunks
 	return s3a.calculateETagFromChunks(entry.Chunks)
 }