Fix package upload for files >32mb (#20622)

* Rewind file before first read.
* Added tests.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/util/filebuffer/file_backed_buffer_test.go

@@ -0,0 +1,36 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package filebuffer
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFileBackedBuffer(t *testing.T) {
+	cases := []struct {
+		MaxMemorySize int
+		Data          string
+	}{
+		{5, "test"},
+		{5, "testtest"},
+	}
+
+	for _, c := range cases {
+		buf, err := CreateFromReader(strings.NewReader(c.Data), c.MaxMemorySize)
+		assert.NoError(t, err)
+
+		assert.EqualValues(t, len(c.Data), buf.Size())
+
+		data, err := io.ReadAll(buf)
+		assert.NoError(t, err)
+		assert.Equal(t, c.Data, string(data))
+
+		assert.NoError(t, buf.Close())
+	}
+}