WebRequest vs FileWebRequest

I am going through the example exam questions for Microsoft exam 70-483 "Programming in C#".

There is one question the answer to which I don't understand and couldn't find anything about the topic on the Internet.

The question is:

You are implementing a method named ProcessFile that retrieves data files from web servers and FTP servers. The ProcessFile() method has the following method signature:
Public void ProcessFile(Guid dataField, string dataFileUri)
Each time the ProcessFile() method is called, it must retrieve a unique data file and then save the data file to disk.
You need to complete the implementation of the ProcessFile() method. Which code segment should you use?

FileWebRequest request = FileWebRequest.Create(dataFileUri) as FileWebRequest;
using (FileWebResponse response = request.GetResponse() as FileWebResponse)
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
using (StreamWriter writer = new StreamWriter(dataFieldId + ".dat"))
{
    writer.Write(reader.ReadToEnd());
}

-or-

WebRequest request = WebRequest.Create(dataFileUri);
using (WebResponse response = request.GetResponse())
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
using (StreamWriter writer = new StreamWriter(dataFieldId + ".dat"))
{
    writer.Write(reader.ReadToEnd());
}

According to the question-making people, the latter snippet, using "WebRequest" is the correct one. But I cannot figure out why the "FileWebRequest" one isn't.
Keep in mind the questions I am doing have been wrong a lot in the past, so maybe this isn't correct, either?

Solution

Some hours ago I also have met this question. Early I have not work with this, but by searching info in Google I concluded followings:

the main words in question is from web servers and FTP servers,

that means that dataFileUri may be like http://mywebserver or ftp://myftpserver
when you try to get file from ftp server, for example:
```
//from answer
var request1 = WebRequest.Create("ftp://myftpserver");   

//from answer
var request2 = FileWebRequest.Create("ftp://myftpserver") as FileWebRequest;   

var request3 = WebRequest.Create("ftp://myftpserver") as FtpWebRequest;
```
request1, request3 will have request value with type SystemNet.FtpWebRequest. request2 will be null.

The similar behavior will be when you try to use http://mywebserver: request2, request3 will be null.
when you use WebRequest the type of the request will be automatically detected by transfer protocol

So you don't need to think about whether file stored on web server/file server/ftp server