in_array() vs if else on file uploading in php - php

Question

Description :
I have given the interface to the user where he/she can upload pdf,doc,docx,img files perfectly.
Every thing is done except I have a confusion on one little piece of code. So before knowing any reason why its done I cant just write it in my php.
While uploading any file at first we do check the type of file before taking any further actions.
Now there are two ways by which we can check one is
$ext = end(explode(".",$_FILES['upload'][name]));
if(in_array($ext,$array_name_of_all_allowed_extensions))
OR
if($_FILES['upload']['type'] == "application/pdf")
{
//any action
}
but every where I see the code for uploading the file is as
$allowedExts = array("pdf", "doc", "docx");
$extension = end(explode(".", $_FILES["file"]["name"]));
if ((($_FILES["file"]["type"] == "application/pdf")
|| ($_FILES["file"]["type"] == "application/msword"))
&& ($_FILES["file"]["size"] < 20000000)
&& in_array($extension, $allowedExts))
{
if ($_FILES["file"]["error"] > 0)
{
echo "Return Code: " . $_FILES["file"]["error"] . "<br>";
}
every one seems to be using the both .. the if condition for the type and in_array() as well.
Can any one tell me why is that. I mean using any one of them in_array() preferred should do the job

Answer 1

Is this understandable :
$ext = end(explode(".",$_FILES['upload'][name]));
if(in_array($ext,$array_name_of_all_allowed_extensions))
the above piece of code my fellow programmer is to check the extension of the file user uploaded it can be any weird extension that you don't your web app to upload so you define an array that only the files that end with ".jpeg , .jpg , .pdf" etc are allowed to be uploaded
if ((($_FILES["file"]["type"] == "application/pdf")
this above line of code checks the type of uploaded file that whether it is the type you allow or not
you can write .exe at the end of any pdf file and it will update the name of the file to mypdf.exe how ever the type of the file will still remain pdf document and it will open as regular as it should ... So checking through if is just a little work that the file being uploaded has a correct name and not been tampered with ...
Its entirely your choice otherwise if you make an array of the allowed file type and then use in_array() that is enough too ... hope that helps :) ile that whether