fix: multipart replication with single part objects (#20895)

x-amz-checksum-algorithm is not set, causing all multipart single-part objects
to fail to replicate going via sftp/FTP uploads.

cmd/encryption-v1.go

@@ -1155,16 +1155,17 @@ func (o *ObjectInfo) metadataEncryptFn(headers http.Header) (objectMetaEncryptFn
 
 // decryptChecksums will attempt to decode checksums and return it/them if set.
 // if part > 0, and we have the checksum for the part that will be returned.
-func (o *ObjectInfo) decryptChecksums(part int, h http.Header) map[string]string {
+// Returns whether the checksum (main part 0) is a multipart checksum.
+func (o *ObjectInfo) decryptChecksums(part int, h http.Header) (cs map[string]string, isMP bool) {
 	data := o.Checksum
 	if len(data) == 0 {
-		return nil
+		return nil, false
 	}
 	if part > 0 && !crypto.SSEC.IsEncrypted(o.UserDefined) {
 		// already decrypted in ToObjectInfo for multipart objects
 		for _, pi := range o.Parts {
 			if pi.Number == part {
-				return pi.Checksums
+				return pi.Checksums, true
 			}
 		}
 	}
@@ -1174,7 +1175,7 @@ func (o *ObjectInfo) decryptChecksums(part int, h http.Header) map[string]string
 			if err != crypto.ErrSecretKeyMismatch {
 				encLogIf(GlobalContext, err)
 			}
-			return nil
+			return nil, part > 0
 		}
 		data = decrypted
 	}