Found the culprit. Line 13 in lib/inc/inc.filters.php:
$excluded = preg_match('/\d{2,}[Xx]\d{2,}\./', $file) ? true : false;
I simply commented it out, as it didn’t work as intended either*.
*This is due to me using customdata, which produces thumbnail filenames like _thumb_lrg_wd_TESTIMG_300x300 [TEST,4 okt 2016].jpg
.