diff --git a/src/Program.cs b/src/Program.cs
index 754b8fd..241e2fb 100644
--- a/src/Program.cs
+++ b/src/Program.cs
@@ -33,7 +33,7 @@ app.UseStaticFiles(new StaticFileOptions
 
 app.UseStatusCodePages();
 
-app.MapPost("/Files/{fileId}", (string fileId, FileService fileService) =>
+app.MapPost("/Files/{fileId}", (string fileId, CreateFileRequest request, FileService fileService) =>
 {
   if (fileService.IsValidFileId(fileId) is false)
   {
@@ -49,6 +49,10 @@ app.MapPost("/Files/{fileId}", (string fileId, FileService fileService) =>
 
   Directory.CreateDirectory(fileDirectory);
 
+  var partFilePath = Path.Combine(fileDirectory, "upload.part");
+  using var handle = File.OpenHandle(partFilePath, FileMode.CreateNew, FileAccess.Write, FileShare.ReadWrite);
+  RandomAccess.SetLength(handle, request.Size);
+
   return Results.Ok(new SessionResponse(fileId, "New upload session created."));
 });
 
@@ -117,7 +121,10 @@ app.MapPost("/Files/{fileId}/upload", async (
   }
 
   using var formStream = chunk.OpenReadStream();
-  var hashBytes = await SHA256.HashDataAsync(formStream);
+  var buffer = new byte[chunk.Length];
+  await formStream.ReadExactlyAsync(buffer);
+
+  var hashBytes = SHA256.HashData(buffer);
   var calculatedHash = Convert.ToHexString(hashBytes).ToLowerInvariant();
 
   if (CryptographicOperations.FixedTimeEquals(Encoding.UTF8.GetBytes(calculatedHash), Encoding.UTF8.GetBytes(hash.ToLowerInvariant())) is false)
@@ -128,20 +135,15 @@ app.MapPost("/Files/{fileId}/upload", async (
   var partFilePath = Path.Combine(fileDirectory, "upload.part");
   long byteOffset = index * chunkSize;
 
-  var fileStreamOptions = new FileStreamOptions
-  {
-    Mode = FileMode.OpenOrCreate,
-    Access = FileAccess.Write,
-    Share = FileShare.ReadWrite,
-    BufferSize = 81920,
-    Options = FileOptions.Asynchronous
-  };
+  using var handle = File.OpenHandle(
+    partFilePath,
+    FileMode.Open,
+    FileAccess.Write,
+    FileShare.ReadWrite,
+    FileOptions.Asynchronous
+  );
 
-  formStream.Position = 0;
-
-  using var targetStream = new FileStream(partFilePath, fileStreamOptions);
-  targetStream.Seek(byteOffset, SeekOrigin.Begin);
-  await formStream.CopyToAsync(targetStream);
+  await RandomAccess.WriteAsync(handle, buffer, byteOffset);
 
   await File.WriteAllTextAsync(markerFilePath, string.Empty);
 
@@ -256,6 +258,7 @@ app.MapDelete("/Files/{fileId}", (string fileId, FileService fileService) =>
 
 app.Run();
 
+record CreateFileRequest(long Size);
 record FileCompleteRequest(string Filename, string CompositeHash);
 record SessionResponse(string FileId, string Message);
 record ChunkStatusInfo(int Index, string Hash);
@@ -263,6 +266,7 @@ record SessionStatusResponse(string FileId, List<ChunkStatusInfo> UploadedChunks
 record ChunkUploadResponse(string FileId, int Index, string Status);
 record CompleteResponse(string Status, string Path);
 
+[JsonSerializable(typeof(CreateFileRequest))]
 [JsonSerializable(typeof(FileCompleteRequest))]
 [JsonSerializable(typeof(SessionResponse))]
 [JsonSerializable(typeof(ChunkStatusInfo))]
diff --git a/src/wwwroot/index.js b/src/wwwroot/index.js
index 1ca26a6..367337b 100644
--- a/src/wwwroot/index.js
+++ b/src/wwwroot/index.js
@@ -56,7 +56,12 @@ document.addEventListener("DOMContentLoaded", () => {
 
     const createFileResponse = await fetch(`/files/${fileId}`, {
       method: "POST",
+      headers: {
+        "content-type": "application/json",
+      },
+      body: JSON.stringify({ size: file.size }),
     });
+
     let existingChunksMap = new Map();
 
     if (createFileResponse.status === 409 || createFileResponse.ok) {