Trying to access array offset on value of type bool

[Arantor]

No, it's a legitimate bug, albeit one that can't readily be reproduced.

[sah62]

I'm still seeing occasional instances of this error with SMF 2.1.4 and PHP 8.1.2. I decided to try a fix by changing this line of code in Subs-Attachments.php:

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// What about the extension?
$thumb_ext = isset($context['valid_image_types'][$size[2]]) ? $context['valid_image_types'][$size[2]] : '';
to this:

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// What about the extension?
$thumb_ext = is_array($size) && isset($context['valid_image_types'][$size[2]]) ? $context['valid_image_types'][$size[2]] : '';
That is, check to make sure that the value returned from getimagesize() and stored in $size is an array before attempting to access the value of $size[2]. If getimagesize() fails and returns boolean false, is_array($size) will evaluate to false and the value of $thumb_ext will be set to ''. That seems safe given that $thumb_ext will also be set to '' if isset($context['valid_image_types'][$size[2]]) evaluates to false.