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"

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"

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

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

Or:

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

It is worth to mention how to remove the extension also in parallel with getting the extension:

var name = Path.GetFileNameWithoutExtension(fileFullName); // Get the name only

var extension = Path.GetExtension(fileFullName); // Get the extension only

You will not be able to restrict the file type that the user uploads at the client side[*]. You'll only be able to do this at the server side. If a user uploads an incorrect file you will only be able to recognise that once the file is uploaded uploaded. There is no reliable and safe way to stop a user uploading whatever file format they want.

[*] yes, you can do all kinds of clever stuff to detect the file extension before starting the upload, but don't rely on it. Someone will get around it and upload whatever they like sooner or later.

You can check .flv signature. You can download specification here:

http://www.adobe.com/devnet/flv/

See "The FLV header" chapter.

private string GetExtension(string attachment_name)
{
    var index_point = attachment_name.IndexOf(".") + 1;
    return attachment_name.Substring(index_point);
}

Path.GetExtension(file.FileName)) will get you the file name

Im also sharing a test code if someone needs to test and ge the extention or name.

Forming a text file with name test.txt and checking its extention in xUnit.

        [Fact]
        public void WrongFileExtention_returnError()
        {
            //Arrange
            string expectedExtention = ".csv";
            var content = "Country,Quantity\nUnited Kingdom,1";
            var fileName = "test.csv";
            var stream = new MemoryStream();
            var writer = new StreamWriter(stream);
            writer.Write(content);
            writer.Flush();
            stream.Position = 0;

            //Act 
            IFormFile file = new FormFile(stream, 0, stream.Length, "", fileName);
            
            //Assert
            Assert.Equal(expectedExtention, Path.GetExtension(file.FileName));

        }

Return true as the expected and the filename extention is name.

Hope this helps someone :).

I know this is quite an old question but here's a nice article on getting the file extension as well as a few more values:

Get File Extension in C#

I Hope That Helps :-)!