"Undefined index: data" error in php - php

Question

I have the following form that allows user to upload the image:
<form name='form1' method="POST" action="" id="registration_form">
<input type="file" name="data" id="data">
<input type="Submit" name="Upload1" value="Upload1">
</form>
And following php code that does the actual upload:
<?php if(isset($_POST['Upload1']))
{
$errors=array();
$allowed_ext= array('pdf','jpg','jpeg');
$file_name =$_FILES['data']['name']; //error shows in this line
$file_ext = strtolower( end(explode('.',$file_name)));
$file_size=$_FILES['data']['size'];
$file_tmp= $_FILES['data']['tmp_name'];
$type = pathinfo($file_tmp, PATHINFO_EXTENSION);
$data = file_get_contents($file_tmp);
if(in_array($file_ext,$allowed_ext) === false){
$errors[]='Extension not allowed';
}
if($file_size > 2097152){
$errors[]= 'File size must be under 2mb';
}
if(empty($errors))
{
if( move_uploaded_file($file_tmp, 'images/'.$file_name));
{
$ct='application/' .$file_ext.'<br/>' ;
$base64 = base64_encode($data);
//json part is here
}
}
else {
foreach($errors as $error) {
echo $error , '<br/>';
}
}
}
When I select the file and hit "UPLOAD", it shows error as "Undefined index: data". But what is interesting to note is that 1 out of 10 times, it works! Where am I doing wrong?

Answer 1

According to your form you are not using multiparty/form-data in form tag.
You just need to use
enctype="multipart/form-data" 
Inside the form tag for input type file.
Example:
<form enctype="multipart/form-data">
Why it's needed? Why is form enctype=multipart/form-data required when uploading a file?
If you are using JavaScript for submit the form than you can use formData for getting data from input either post or file.
var formData = new FormData(formId);