title: How to check if a file contains html instead of true content, when using a .php download service?
body: <p>My C# WinForms app downloads files from a .php downloading web service.</p>
  
  <p>The .php page returns error as HTML pages displayed to the user (I don't have an alternative API for this)</p>
  
  <p>What would be the best way, provided a file on the disk, to:
     1) identify if the file is an HTML file.
     2) determine if it has the content of an error page (specified as comment tags, or plain text displayed to the user)</p>
  
  <p>Thanks in advance.</p>