Added Upload method with large files

Signed-off-by: kmohan <kmohan@sujainfo.net>

Models/Base64EncodingStream .cs

+using System.Text;
+
+public class Base64EncodingStream : Stream
+{
+    private readonly Stream _innerStream;
+    private readonly MemoryStream _base64BufferStream = new MemoryStream();
+    private bool _endOfStream = false;
+
+    public Base64EncodingStream(Stream innerStream)
+    {
+        _innerStream = innerStream ?? throw new ArgumentNullException(nameof(innerStream));
+    }
+
+    public override bool CanRead => true;
+    public override bool CanSeek => false;
+    public override bool CanWrite => false;
+    public override long Length => throw new NotSupportedException();
+    public override long Position
+    {
+        get => throw new NotSupportedException();
+        set => throw new NotSupportedException();
+    }
+
+    public override void Flush() => throw new NotSupportedException();
+
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        if (_endOfStream && _base64BufferStream.Length == 0)
+            return 0; // End of stream
+
+        // Fill the Base64 buffer if it's empty
+        if (_base64BufferStream.Length == 0)
+        {
+            byte[] readBuffer = new byte[81920]; // 80 KB
+            int bytesRead = _innerStream.Read(readBuffer, 0, readBuffer.Length);
+
+            if (bytesRead > 0)
+            {
+                string base64Chunk = Convert.ToBase64String(readBuffer, 0, bytesRead);
+                byte[] base64Bytes = Encoding.UTF8.GetBytes(base64Chunk);
+                _base64BufferStream.Write(base64Bytes, 0, base64Bytes.Length);
+                _base64BufferStream.Position = 0; // Reset position for reading
+            }
+            else
+            {
+                _endOfStream = true;
+            }
+        }
+
+        // Read Base64 data from the buffer
+        int bytesToCopy = Math.Min(count, (int)(_base64BufferStream.Length - _base64BufferStream.Position));
+        int bytesCopied = _base64BufferStream.Read(buffer, offset, bytesToCopy);
+
+        // If the Base64 buffer is exhausted, reset it
+        if (_base64BufferStream.Position == _base64BufferStream.Length)
+        {
+            _base64BufferStream.SetLength(0);
+            _base64BufferStream.Position = 0;
+        }
+
+        return bytesCopied;
+    }
+
+    public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
+    public override void SetLength(long value) => throw new NotSupportedException();
+    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
+
+    protected override void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            _innerStream?.Dispose();
+            _base64BufferStream?.Dispose();
+        }
+        base.Dispose(disposing);
+    }
+}