plugin meta box generates warning log when using file_input

hello,

thank you for this awesome plugin,

I want to report an issue when using the "file_input" type.

the error is:

[30-Mar-2022 09:04:35 UTC] PHP Notice:  Only variables should be passed by reference in C:\fakepath\wp-content\plugins\meta-box\inc\fields\file-input.php on line 38

after I look at file-input.php on line 38, I change it 
<strong>from</strong>

<blockquote>$file_ext   = strtolower( end( explode( '.', $meta ) ) );</blockquote>

<strong>to </strong>

<blockquote>$end = explode( '.', $meta );
$file_ext   = strtolower( end( $end ) );</blockquote>

the error disappears.

I hope this issue will be patched asap, thank you

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