Checking file type from base64?

Question

I have a WCF REST Service with the following OperationContract that saves files on the disk:

[OperationContract]
[WebInvoke(UriTemplate = "FileSave", Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.WrappedRequest)]
ResponseHandler FileSave(string fileName, string fileContent);

Files are sent through javascript - using HTML File API => binary data => base-64 encoded ASCII string (=fileContent is recieved in the operation contract)

I want to check the file type before saving the file on the disk. I am aware of Checking MIME Type from a base64 string on the Code Review Stack Exchange but I am not sure if it is the best way to go. Also, I have tested uploading several .txt files and each one has different first 5 chars.

I am looking for a code snippet that would include checking for several common file types.

Answer 1

Check this link here:

https://web.archive.org/web/20170331115315/http://codeanalyse.com/2016/10/02/extracting-file-extension-base64-string/

This "would include checking for several common file types"

/// <summary>
/// To demonstrate extraction of file extension from base64 string.
/// </summary>
/// <param name="base64String">base64 string.</param>
/// <returns>Henceforth file extension from string.</returns>
public static string GetFileExtension(string base64String)
{
var data = base64String.Substring(0, 5);

switch (data.ToUpper())
 {
     case "IVBOR":
        return "png";
     case "/9J/4":
         return "jpg";
     case "AAAAF":
         return "mp4";
     case "JVBER":
         return "pdf";
     case "AAABA":
         return "ico";
     case "UMFYI":
         return "rar";
     case "E1XYD":
         return "rtf";
     case "U1PKC":
        return "txt";
     case "MQOWM":
     case "77U/M":
        return "srt";
     default:
        return string.Empty;
 }
}

Answer 2

Trying to figure out the file type by checking the file content is always prone to errors - you don't know all possible file types, file headers change etc...

Just do it the either the way browsers do it - by the mime type: In javascript, check the file type via the HTML File API (evt.dataTransfer.files[0].type), then send that as part of your JSON message to the server

Or do it the way windows does it - by the file name extension.