Adding proper open support for memfile

memfile.go

diff --git a/memfile.go b/memfile.go
index ceaf528..fcf80d2 100644
--- a/memfile.go
+++ b/memfile.go
@@ -45,6 +45,11 @@
 func MemFileCreate(name string) *InMemoryFile {
 	return &InMemoryFile{name: name, mode: os.ModeTemporary, modtime: time.Now()}
 }
+
+func (f *InMemoryFile) Open() error {
+	atomic.StoreInt64(&f.at, 0)
+	f.closed = false
+	return nil
 }
 
 func (f *InMemoryFile) Close() error {

memmap.go

diff --git a/memmap.go b/memmap.go
index af3c653..d820502 100644
--- a/memmap.go
+++ b/memmap.go
@@ -152,9 +152,13 @@
 func (m *MemMapFs) Open(name string) (File, error) {
 	m.rlock()
 	f, ok := m.getData()[name]
-	m.runlock()
+	ff, ok := f.(*InMemoryFile)
 	if ok {
-		f.Seek(0, 0)
+		ff.Open()
+	}
+	m.runlock()
+
+	if ok {
 		return f, nil
 	} else {
 		return nil, ErrFileNotFound