'ControllerBase.File(byte[], string)' is a method, which is not valid in the given context (CS0119) - in method

I am trying to create an app where user can upload a text file, and gets the altered text back.

I am using React as FE and ASP.NET Core for BE and Azure storage for the database storage.

This is how my HomeController looks like. I created a separate "UploadToBlob" method, to post the data

    public class HomeController : Controller
    {
        private readonly IConfiguration _configuration;

        public HomeController(IConfiguration Configuration)
        {
            _configuration = Configuration;
        }

        public IActionResult Index()
        {
            return View();
        }

        [HttpPost("UploadFiles")]
        //OPTION B: Uncomment to set a specified upload file limit
        [RequestSizeLimit(40000000)]

        public async Task<IActionResult> Post(List<IFormFile> files)
        {
            var uploadSuccess = false;
            string uploadedUri = null;

            foreach (var formFile in files)
            {
                if (formFile.Length <= 0)
                {
                    continue;
                }

                // read directly from stream for blob upload      
                using (var stream = formFile.OpenReadStream())
                {
                    // Open the file and upload its data
                    (uploadSuccess, uploadedUri) = await UploadToBlob(formFile.FileName, null, stream);

                }

            }

            if (uploadSuccess)
            {
                //return the data to the view, which is react display text component.
                return View("DisplayText");
            }
            else
            {
                //create an error component to show there was some error while uploading
                return View("UploadError");
            }
        }

        private async Task<(bool uploadSuccess, string uploadedUri)> UploadToBlob(string fileName, object p, Stream stream)
        {
            if (stream is null)
            {
                try
                {
                    string connectionString = Environment.GetEnvironmentVariable("AZURE_STORAGE_CONNECTION_STRING");

                    // Create a BlobServiceClient object which will be used to create a container client
                    BlobServiceClient blobServiceClient = new BlobServiceClient(connectionString);

                    //Create a unique name for the container
                    string containerName = "textdata" + Guid.NewGuid().ToString();

                    // Create the container and return a container client object
                    BlobContainerClient containerClient = await blobServiceClient.CreateBlobContainerAsync(containerName);

                    string localPath = "./data/";
                    string textFileName = "textdata" + Guid.NewGuid().ToString() + ".txt";
                    string localFilePath = Path.Combine(localPath, textFileName);

                    // Get a reference to a blob
                    BlobClient blobClient = containerClient.GetBlobClient(textFileName);

                    Console.WriteLine("Uploading to Blob storage as blob:\n\t {0}\n", blobClient.Uri);

                    FileStream uploadFileStream = File.OpenRead(localFilePath);
                    await blobClient.UploadAsync(uploadFileStream, true);
                    uploadFileStream.Close();
                }
                catch (StorageException)
                {
                    return (false, null);
                }
                finally
                {
                    // Clean up resources, e.g. blob container
                    //if (blobClient != null)
                    //{
                    //    await blobClient.DeleteIfExistsAsync();
                    //}
                }
            }
            else
            {
                return (false, null);
            }

        }

    }

but the console throws errors, saying "'ControllerBase.File(byte[], string)' is a method, which is not valid in the given context (CS0119)"

And because of this error, another error follows "'HomeController.UploadToBlob(string, object, Stream)': not all code paths return a value (CS0161)"

my questions are

1. Is it a better idea to create a separate method like I did?
2. how can I resolve the issue regarding the "File" being valid inside of the UploadToBlob method?
3. If I want to add the file type validation, where should it happen? t.ex. only text file is alid
4. If I want to read the text string from the uploaded text file, where should I call the

  string contents = blob.DownloadTextAsync().Result;

  return contents;

1. How can I pass down the "contents" to my react component? something like this?

    useEffect(() => {
        fetch('Home')
            .then(response => response.json())
            .then(data => {
                setForcasts(data)
            })
    }, [])

Thanks for helping this super newbie with ASP.NET Core!

FileStream uploadFileStream = System.IO.File.OpenRead(localFilePath);

private Stream FindMostFrequentWordAndReplaceIt(Stream inputStream)
{
    using (var sr = new StreamReader(inputStream, Encoding.UTF8)) // what is the encoding of the text? 
    {
        var allText = sr.ReadToEnd(); // read all text into memory
        // TODO: Find most frequent word in allText
        // replace the word allText.Replace(oldValue, newValue, stringComparison)
        var resultText = allText.Replace(...);

        var result = new MemoryStream();
        using (var sw = new StreamWriter(result))
        {
            sw.Write(resultText);
        }
        result.Position = 0;
        return result;
    }
}

using (var stream = formFile.OpenReadStream())
{
    var streamWithReplacement = FindMostFrequentWordAndReplaceIt(stream);

    // Upload the replaced text:
    (uploadSuccess, uploadedUri) = await UploadToBlob(formFile.FileName, null, streamWithReplacement);

}