How to check file type exists or not from list

Here is the logic you can use as your requirement 

var supportedTypes = new[] { "txt", "doc", "docx", "pdf", "xls", "xlsx" };

var fileExt = System.IO.Path.GetExtension(file.FileName).Substring(1);

Here, GetExtension method is used to get the extension of uploaded file that is file.FileName From the path of the system. 

if (!supportedTypes.Contains(fileExt))
{
ErrorMessage = "File Extension Is InValid - Only Upload WORD/PDF/EXCEL/TXT File"; return ErrorMessage;
}