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/fs-v1-metadata.go

@@ -29,6 +29,8 @@ import (
 	"time"
 
 	jsoniter "github.com/json-iterator/go"
+	"github.com/minio/minio/internal/crypto"
+	"github.com/minio/minio/internal/etag"
 	xhttp "github.com/minio/minio/internal/http"
 	"github.com/minio/minio/internal/lock"
 	"github.com/minio/minio/internal/logger"
@@ -167,6 +169,12 @@ func (m fsMetaV1) ToObjectInfo(bucket, object string, fi os.FileInfo) ObjectInfo
 	}
 
 	objInfo.ETag = extractETag(m.Meta)
+
+	et, e := etag.Parse(objInfo.ETag)
+	if e == nil {
+		objInfo.Multipart = et.IsMultipart()
+	}
+
 	objInfo.ContentType = m.Meta["content-type"]
 	objInfo.ContentEncoding = m.Meta["content-encoding"]
 	if storageClass, ok := m.Meta[xhttp.AmzStorageClass]; ok {
@@ -176,7 +184,6 @@ func (m fsMetaV1) ToObjectInfo(bucket, object string, fi os.FileInfo) ObjectInfo
 	}
 	var (
 		t time.Time
-		e error
 	)
 	if exp, ok := m.Meta["expires"]; ok {
 		if t, e = time.Parse(http.TimeFormat, exp); e == nil {
@@ -193,6 +200,14 @@ func (m fsMetaV1) ToObjectInfo(bucket, object string, fi os.FileInfo) ObjectInfo
 	// Tags have also been extracted, we remove that as well.
 	objInfo.UserDefined = cleanMetadata(m.Meta)
 
+	// Set multipart for encryption properly if
+	// not set already.
+	if !objInfo.Multipart {
+		if _, ok := crypto.IsEncrypted(objInfo.UserDefined); ok {
+			objInfo.Multipart = crypto.IsMultiPart(objInfo.UserDefined)
+		}
+	}
+
 	// All the parts per object.
 	objInfo.Parts = m.Parts