The JavaScript script from this tutorial checks automatically the name of the file which an user adds for upload.
You can add /change the list of allowed extensions in the "ar_ext" variable.
The script separate the name and the extension of the file, adds the name in a text field and checks the extension.
If the file extension is found in the allowed list, enables the "Submit" button, else, it deletes the value in the "file" field, disables the Submit button, and displays an alert message.
<form action="upload.php" method="post" id="idf" enctype="multipart/form-data">
 Upload file: <input type="file" name="fup" onchange="checkName(this, 'fname', 'submit')" /><br />
 File name: <input type="text" value="" name="denumire" id="fname" /><br />
 <input type="submit" id="submit" value="Submit" disabled="disabled" />
</form>
<script type="text/javascript"><!--
var ar_ext = ['pdf', 'gif', 'jpe', 'jpg'];        // array with allowed extensions

function checkName(el, to, sbm) {
// - coursesweb.net
  // get the file name and split it to separe the extension
  var name = el.value;
  var ar_name = name.split('.');

  // for IE - separe dir paths (\) from name
  var ar_nm = ar_name[0].split('\\');
  for(var i=0; i<ar_nm.length; i++) var nm = ar_nm[i];

  // add the name in 'to'
  document.getElementById(to).value = nm;

  // check the file extension
  var re = 0;
  for(var i=0; i<ar_ext.length; i++) {
    if(ar_ext[i] == ar_name[1]) {
      re = 1;
      break;
    }
  }

  // if re is 1, the extension is in the allowed list
  if(re==1) {
    // enable submit
    document.getElementById(sbm).disabled = false;
  }
  else {
    // delete the file name, disable Submit, Alert message
    el.value = '';
    document.getElementById(sbm).disabled = true;
    alert('".'+ ar_name[1]+ '" is not an file type allowed for upload');
  }
}
--></script>
The "upload.php" files must contain a PHP script which will get the file and copy it on the server. Bellow is the code for "upload.php" file.

upload.php code

<?php
// - coursesweb.net
$updir = 'uploads/';       // sets the folder where the uploaded files are copied
$max_size = 100;         // sets maximum file size allowed (in KB)

// file types allowed
$allowtype = array('pdf', 'gif', 'jpg', 'jpe');

// If is received a valid file from the 'fup' form field
if (isset($_FILES['fup'])) {
  // check for errors
  if ($_FILES['fup']['error'] > 0) {
    echo 'Error: '. $_FILES['fup']['error']. '<br />';
  }
  else {
    // get the name, size (in kb) and type (the extension) of the file
    $fname = $_FILES['fup']['name'];
    $fsize = $_FILES['fup']['size'] / 1024;
    $ftype = end(explode('.', strtolower($fname)));

    // checks if the file already exists
    if (file_exists($updir. $fname)) {
      echo 'The file: '. $fname. ' already exists';
    }
    else {
      // if the file not exists, check its type (by extension) and size
      if (in_array($ftype, $allowtype)) {
        // check the size
        if ($fsize <= $max_size) {
          // uses  function to copy the file from temporary folder to $updir
          if (!move_uploaded_file ($_FILES['fup']['tmp_name'], $updir. $fname)) {
            echo 'The file '. $fname. ' could not be copied, try again';
          }
          else {
            echo $fname. ' ('. $fsize. ' kb) was successfully uploaded';
          }
        }
        else {
          echo 'The file '. $fname. ' exceeds the maximum permitted size, '. $max_size. ' KB';
        }
      }
      else {
        echo $fname. ' - invalid file type';
      }
    }
  }
}
else echo 'Invalid form data';
?>

You can test the JavaScript script in the next form (file types allowed: PDF, GIF, JPE, and JPG):
Upload file:
File name:

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which tag is used in <table> to create table header cell?
<thead> <th> <td>
<table><tr>
  <th>Title 1</th>
  <th>Title 2</th>
</tr></table>
Which CSS property sets the distance between lines?
line-height word-spacing margin
.some_class {
  line-height: 150%;
}
Which function opens a new browser window.
alert() confirm() open()
document.getElementById("id_button").onclick = function(){
  window.open("http://coursesweb.net/");
}
Indicate the PHP function that returns an array with names of the files and folders inside a directory.
mkdir() scandir() readdir()
$ar_dir = scandir("dir_name");
var_export($ar_dir);
Check the file type before Upload

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