Http posted file save as

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/Ref.Standard.cs

@@ -313,6 +313,7 @@ internal HttpPostedFile() { }
         public string ContentType { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public string FileName { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public System.IO.Stream InputStream { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
+        public void SaveAs(string filename) { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");}
     }
     public abstract partial class HttpPostedFileBase
     {

src/Microsoft.AspNetCore.SystemWebAdapters/HttpPostedFile.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Globalization;
 using System.IO;
 using Microsoft.AspNetCore.Http;
 
@@ -19,4 +20,20 @@ public sealed class HttpPostedFile
     public int ContentLength => (int)File.Length;
 
     public Stream InputStream => File.OpenReadStream();
+
+    public void SaveAs(string filename)
+    {
+
+        if (!Path.IsPathRooted(filename))
+        {
+            throw new HttpException(string.Format(CultureInfo.InvariantCulture,
+                "The SaveAs method is configured to require a rooted path, and the path '{0}' is not rooted", filename));
+        }
+
+        using (var fileStream = new FileStream(filename, FileMode.Create))
+        {
+            InputStream.Seek(0, SeekOrigin.Begin);
+            InputStream.CopyTo(fileStream);
+        }
+    }
 }

test/Microsoft.AspNetCore.SystemWebAdapters.Tests/HttpPostedFileTests.cs

@@ -100,4 +100,16 @@ public void InputStream()
         // Assert
         Assert.Equal(expected.Object, stream);
     }
+
+    [Fact]
+    public void SaveAs()
+    {
+        var file = new Mock<IFormFile>();
+        var expectedStream = new Mock<Stream>();
+        file.Setup(f => f.OpenReadStream()).Returns(expectedStream.Object);
+
+        var posted = new HttpPostedFile(file.Object);
+        Assert.Throws<HttpException>(() => posted.SaveAs("InvalidPath"));
+    }
+
 }