Fix issue #1654, if XSSFWorkbook stream position is greater than zero, FileMagic.ValueOf returns the correct value

main/POIFS/FileSystem/FileMagic.cs

@@ -141,28 +141,30 @@ public static FileMagic ValueOf(byte[] magic)
         {
             foreach(var fm in Values)
             {
-                int i=0;
-                bool found = true;
                 foreach(byte[] ma in fm.Value.magic)
                 {
-                    foreach(byte m in ma)
-                    {
-                        byte d = magic[i++];
-                        if(!(d == m || (m == 0x70 && (d == 0x10 || d == 0x20 || d == 0x40))))
-                        {
-                            found = false;
-                            break;
-                        }
-                    }
-                    if(found)
+                    if(FindMagic(ma, magic))
                     {
                         return fm.Key;
                     }
                 }
             }
             return FileMagic.UNKNOWN;
         }
-
+        private static bool FindMagic(byte[] expected, byte[] actual)
+        {
+            int i=0;
+            foreach(byte expectedByte in expected)
+            {
+                byte actualByte = actual[i++];
+                if((actualByte != expectedByte &&
+                        (expectedByte != 0x70 || (actualByte != 0x10 && actualByte != 0x20 && actualByte != 0x40))))
+                {
+                    return false;
+                }
+            }
+            return true;
+        }
         /**
          * Get the file magic of the supplied InputStream (which MUST
          *  support mark and reset).<p>

main/Util/IOUtils.cs

@@ -113,6 +113,10 @@ public static byte[] PeekFirstNBytes(Stream stream, int limit)
             {
                 is1.Mark(limit);
             }
+            else
+            {
+                stream.Seek(0, SeekOrigin.Begin);
+            }
             ByteArrayOutputStream bos = new ByteArrayOutputStream(limit);
             if(stream is ByteArrayInputStream inputStream)
                 Copy(new BoundedInputStream(inputStream, limit), bos);
@@ -146,7 +150,7 @@ public static byte[] PeekFirstNBytes(Stream stream, int limit)
             }
             else
             {
-                stream.Position = mark;
+                stream.Seek(mark, SeekOrigin.Begin);
             }
 
             return peekedBytes;

testcases/ooxml/POIFS/FileSystem/TestFileMagic.cs

@@ -0,0 +1,47 @@
+/* ====================================================================
+   Licensed to the Apache Software Foundation (ASF) under one or more
+   contributor license agreements.  See the NOTICE file distributed with
+   this work for Additional information regarding copyright ownership.
+   The ASF licenses this file to You under the Apache License, Version 2.0
+   (the "License"); you may not use this file except in compliance with
+   the License.  You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+==================================================================== */
+
+using NPOI.POIFS.FileSystem;
+using NPOI.XSSF.UserModel;
+using NUnit.Framework;
+using NUnit.Framework.Legacy;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace TestCases.POIFS.FileSystem
+{
+    [TestFixture]
+    public class TestFileMagic
+    {
+        [Test]
+        public void TestXSSFWorkbookStreamPositionGreaterThanZero()
+        {
+            var workbook = new XSSFWorkbook();
+            workbook.CreateSheet("Nothing here");
+            var stream = new MemoryStream();
+            workbook.Write(stream, leaveOpen: true);
+            ClassicAssert.AreEqual(FileMagic.OOXML, FileMagicContainer.ValueOf(stream));
+            stream.Position = 1;
+            ClassicAssert.AreEqual(FileMagic.OOXML, FileMagicContainer.ValueOf(stream));
+            workbook.Close();
+        }
+    }
+}