You cannot include the data:URI scheme (data:image/png;base64,) inside $data.
You can remove the data:URI scheme using the explode function, which will return an array with two elements, and before use base64_decode.
$data[0] => the data:URI scheme.
$data[1] => your data.
Use it as follow, using , as delimiter. Example:
list($dataUriScheme, $data) = explode(',', $data);
Or without list:
$data = explode(',', $data)[1];
And then Try the next code:
$fileName = Input::get('file_name');
$fileStream = fopen(public_path() . '/user_attachments/' . $fileName , "wb");
fwrite($fileStream, base64_decode($data));
fclose($fileStream);
or
// The final filename.
$fileName= Input::get('file_name');
// Upload path
$uploadPath = public_path() . "/user_attachments/" . $fileName;
// Decode your image/file
$data = base64_decode($data);
// Upload the decoded file/image
if(file_put_contents($uploadPath , $data)){
echo "Success!";
}else{
echo "Unable to save the file.";
}