5.7.2 always get same error when using $fi->import() function

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i am using this code:
<?php
Loader::library("file/importer");
$fi = new FileImporter();
$url = 'https://www.gravatar.com/avatar/874b378607eb21bc868bdadb8847560f?s=32&d=identicon&r=PG';
//file_get_contents($url)
$newFile = $fi->import($_FILES['my_image']['tmp_name']);
//echo $newFile." 123";
print FileImporter::getErrorMessage($newFile);
?>
<form method="post" action="" enctype="multipart/form-data">
   <input type="file" name="my_image"/>
   <input type="submit">
</form>
<pre>
<?php print_r($_FILES); ?>

to upload and stroe the file in the file manager.
i always get an error "Invalid file extension." i think this may me a bug.i have uploaded all kinds of files always get same error.
that being said i can upload files via the file manager no problem.
v- 5.7.2