How to find the extension of a file in C#?

Path.GetExtension

string myFilePath = @"C:\MyFile.txt";
string ext = Path.GetExtension(myFilePath);
// ext would be ".txt"

You may simply read the stream of a file

using (var target = new MemoryStream())
{
    postedFile.InputStream.CopyTo(target);
    var array = target.ToArray();
}

First 5/6 indexes will tell you the file type. In case of FLV its 70, 76, 86, 1, 5.

private static readonly byte[] FLV = { 70, 76, 86, 1, 5};

bool isAllowed = array.Take(5).SequenceEqual(FLV);

if isAllowed equals true then its FLV.

Read the content of a file

var contentArray = target.GetBuffer();
var content = Encoding.ASCII.GetString(contentArray);

First two/three letters will tell you the file type.
In case of FLV its "FLV......"

content.StartsWith("FLV")

At the server you can check the MIME type, lookup flv mime type here or on google.

You should be checking that the mime type is

video/x-flv

If you were using a FileUpload in C# for instance, you could do

FileUpload.PostedFile.ContentType == "video/x-flv"

string FileExtn = System.IO.Path.GetExtension(fpdDocument.PostedFile.FileName);

The above method works fine with the Firefox and IE: I am able to view all types of files like zip,txt,xls,xlsx,doc,docx,jpg,png.

But when I try to find the extension of file from Google Chrome, I fail.

I'm not sure if this is what you want but:

Directory.GetFiles(@"c:\mydir", "*.flv");

Or:

Path.GetExtension(@"c:\test.flv")

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