Upload a file with specific filename

For me to easy browse the right file before uploading, this what i want to accomplish. How to do that?

My input codes: Inside the form are two inputs. And with likely similar filename. Submit button will trigger the "uploadnow" function.

 <td>UPLOAD#1: AGL_001.txt <input type="file" name="upload1" id="upload1"></td>
 <td>UPLOAD#2: AGL_0001.txt <input type="file" name="upload2" id="upload2"></td>

function uploadnow(){
   $allowed_upload1 = ['AGL_001.txt']; // added
   $allowed_upload2 = ['AGL_0001.txt']; //added

    if(isset($_FILES['upload1']['name'])){
        //$errors= array();
        $file_name = $_FILES['upload1']['name'];
        $file_size =$_FILES['upload1']['size'];
        $file_tmp =$_FILES['upload1']['tmp_name'];
        $file_type=$_FILES['upload1']['type'];
        $file_ext=strtolower(end(explode('.',$_FILES['upload1']['name'])));
        //$img_loc = $file_name.'.'.$file_ext;


      if (in_array($file_name, $allowed_upload1)) {
        move_uploaded_file($file_tmp,"uploads/".$file_name);
      } else {
        $message = "Sorry, wrong filename on UPLOAD#1";
        echo "<script type='text/javascript'>alert('$message');</script>";
      }
    }

    if(isset($_FILES['upload2']['name'])){
        //$errors= array();
        $file_name = $_FILES['upload2']['name'];
        $file_size =$_FILES['upload2']['size'];
        $file_tmp =$_FILES['upload2']['tmp_name'];
        $file_type=$_FILES['upload2']['type'];
        $file_ext=strtolower(end(explode('.',$_FILES['upload2']['name'])));
        //$img_loc = $file_name.'.'.$file_ext;

      if (in_array($file_name, $allowed_upload2)) {
          move_uploaded_file($file_tmp,"uploads/".$file_name);
      } else {
          $message = "Sorry, wrong filename on UPLOAD#2";
          echo "<script type='text/javascript'>alert('$message');</script>";
      }
    }
}

I believe you can't preset the filename client-side, but you can constrain accepted file types with accept attribute on your input (thought it's tough to predict all mime types .txt comes with):

<input type="file" accept="text/plain">

or more general

<input type="file" accept="text/*">

Though this is not a bullet proof solution and you can foul the browser.

You can implement a JavaScript solution, which will listen to input changes and validate filename.

To do it server side, just check the value of $_FILES[$input_name]['name'] (or $_FILES[$input_name][$index]['name'] for a file input with multiple attribute)

$allowed_filenames = [
   'AGL_001.txt',
];

if(isset($_FILES['upload1']['name'])){
    //$errors= array();
    $file_name = $_FILES['upload1']['name'];
    $file_size =$_FILES['upload1']['size'];
    $file_tmp =$_FILES['upload1']['tmp_name'];
    $file_type=$_FILES['upload1']['type'];
    $file_ext=strtolower(end(explode('.',$_FILES['upload1']['name'])));
    //$img_loc = $file_name.'.'.$file_ext;

    if (in_array($file_name, $allowed_filenames)) {
        move_uploaded_file($file_tmp,"uploads/".$file_name);
    } else {
        // log an error
    }
}