Porting asp to asp.NET (with fileup)

Short answer: the object you get from upl.Form(item) is a plain string (a form field), not a file object, so ContentType does not exist on it. That explains the "Public member 'ContentType' on type 'String' not found" error. @rompaa was on the right track thinking about types and casting, and @jackster is also right that checking the extension is a quick check — but both need to be applied in the right place and with safer validation behind them.

First troubleshooting step: print the runtime type and value for the item you think is a file to confirm what you actually have. For example, in C# you can log the full type name to the response or a trace log:

object o = upl.Form(item);
Response.Write(o == null ? "null" : o.GetType().FullName);

If that shows System.String, you are indexing the wrong collection. Use the upload control's files collection (or ASP.NET Request.Files) to get posted files (those objects expose ContentType, FileName, InputStream, etc.), or cast only after you confirm the object really is a file type (SaFile for SoftArtisans).

For robust validation do three things: (1) check the server-side MIME (file.ContentType) and extension; (2) verify the binary signature (magic bytes) before trusting it; (3) optionally try to decode it with an image library to be certain. A tiny example to check JPEG magic bytes:

bool IsJpeg(Stream s) {
  var b = new byte[3];
  s.Read(b,0,3);
  s.Position = 0;
  return b[0]==0xFF && b[1]==0xD8 && b[2]==0xFF;
}

Final notes: casting helps only when the object is actually a file object; extension checks alone are spoofable; and using a proper files collection (or the control vendor's API for SaFile objects) will avoid the "String" problem altogether.