How to convert a file into byte array in memory?

As opposed to saving the data as a string (which allocates more memory than needed and might not work if the binary data has null bytes in it), I would recommend an approach more like

foreach (var file in files)
{
  if (file.Length > 0)
  {
    using (var ms = new MemoryStream())
    {
      file.CopyTo(ms);
      var fileBytes = ms.ToArray();
      string s = Convert.ToBase64String(fileBytes);
      // act on the Base64 data
    }
  }
}

Also, for the benefit of others, the source code for IFormFile can be found on GitHub

You can just write a simple extension:

public static class FormFileExtensions
{
    public static async Task<byte[]> GetBytes(this IFormFile formFile)
    {
        await using var memoryStream = new MemoryStream();
        await formFile.CopyToAsync(memoryStream);
        return memoryStream.ToArray();
    }
}

Usage

var bytes = await formFile.GetBytes();
var hexString = Convert.ToBase64String(bytes);