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

The Question :

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In my web application (,c#) I am uploading video file in a page but I want to upload only flv videos. How can I restrict when I upload other extension videos?

The Question Comments :
  • If possible, you want to also check on the client, to avoid unnecessary uploads.
  • Yes, but see my answer below – do NOT rely on client side checking – it will be circumvented sooner or later. 🙂
  • The client-side check is not a protection for your server, but a convenience for the user.
  • Granted. I’m just advising the OP not to rely on it.

The Answer 1

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

The Answer 2

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You may simply read the stream of a file

using (var target = new MemoryStream())
    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……”


The Answer 3

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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


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

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

The Answer 4

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

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



The Answer 5

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In addition, if you have a FileInfo fi, you can simply do:

string ext = fi.Extension;

and it’ll hold the extension of the file (note: it will include the ., so a result of the above could be: .jpg .txt, and so on….

The Answer 6

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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.

The Answer 7

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

See “The FLV header” chapter.

The Answer 8

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Found an alternate solution over at DotNetPerls that I liked better because it doesn’t require you to specify a path. Here’s an example where I populated an array with the help of a custom method

        // This custom method takes a path 
        // and adds all files and folder names to the 'files' array
        string[] files = Utilities.FileList("C:\", "");
        // Then for each array item...
        foreach (string f in files)
            // Here is the important line I used to ommit .DLL files:
            if (!f.EndsWith(".dll", StringComparison.Ordinal))
                // then populated a listBox with the array contents

The Answer 9

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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 googlechrome , i failed.

The Answer 10

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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

The Answer 11

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This solution also helps in cases of more than one extension like “Avishay.student.DB”

                FileInfo FileInf = new FileInfo(filePath);
                string strExtention = FileInf.Name.Replace(System.IO.Path.GetFileNameWithoutExtension(FileInf.Name), "");


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