Read content of uploaded file within ItemAdding method

In an application it is required to validate the file content before allowing the upload into a document library. SharePoint 2010 enables this via the ItemAdding synchronous SPItemEventReceiver method. Problem is however that you cannot access the uploaded file through the SPItemEventProperties.ListItem object. The item is at runtime of ItemAdding not yet created in and thus not available via the list. Via blogpost Getting file content from the ItemAdding method of SPItemEventReceiver I found a code-snippet how to access the file. Last remaining issue was that I received empty result upon reading from the filestream. Debugging I discovered that the Stream position was set to the end of file. Which is logical since the file has been read in order to save it to the content database. By resetting the position I can read the file contents, validate it, and cancel the upload in case of invalid content

public override void ItemAdding(SPItemEventProperties properties)
{
  string searchForFileName = Path.GetFileName(properties.BeforeUrl);
  HttpFileCollection collection = _context.Request.Files;
  for (int i = 0; i < collection.Count; i++)
  {
    HttpPostedFile postedFile = collection[i];
    if (searchForFileName.Equals(
      Path.GetFileName(postedFile.FileName), StringComparison.OrdinalIgnoreCase))
    {
      Stream fileStream = postedFile.InputStream;
      fileStream.Position = 0;
      byte[] fileContents = new byte[postedFile.ContentLength];
      fileStream.Read(fileContents, 0, postedFile.ContentLength);
      ...