i got the same problem after upgrading to forum 2.3.1 without running the upgrade script.
here is what i did to solve it.
on line 429 of usersettings.php, look for this
Text Formatted Code
$upload->setAllowedMimeTypes(array('image/gif','image/jpeg','image/pjpeg','image/x-png'))
replace that line with this one.
Text Formatted Code
$upload->setAllowedMimeTypes(array('image/gif' => '.gif','image/jpeg' => '.jpg,.jpeg','image/pjpeg' => 'jpg,.jpeg','image/x-png' => '.png','image/png' => '.png' ));
hope this helps.
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