allow multipart uploads for single part multipart (#12821)

its possible that some multipart uploads would have
uploaded only single parts so relying on `len(o.Parts)`
alone is not sufficient, we need to look for ETag
pattern to be absolutely sure.

cmd/object-api-datatypes.go

@@ -173,6 +173,8 @@ type ObjectInfo struct {
 
 	Legacy bool // indicates object on disk is in legacy data format
 
+	Multipart bool // indicates if object is multipart object.
+
 	// backendType indicates which backend filled this structure
 	backendType BackendType
 
@@ -193,6 +195,7 @@ func (o ObjectInfo) Clone() (cinfo ObjectInfo) {
 		Size:               o.Size,
 		IsDir:              o.IsDir,
 		ETag:               o.ETag,
+		Multipart:          o.Multipart,
 		InnerETag:          o.InnerETag,
 		VersionID:          o.VersionID,
 		IsLatest:           o.IsLatest,