I know, I am not the first guy having this problem, but non of the suggested solutions worked:

defined( 'ABSPATH' ) or die( 'No script kiddies please!' );
function my_custom_upload_mimes($mimes = array()) {

    $mimes['glb'] = "model/gltf-binary";
    return $mimes;
    }
    
    add_action('upload_mimes', 'my_custom_upload_mimes');

I don’t have a multi site. I don’t have any other plugins. I am still not allowed to upload glb files:

yourfile.glb Sorry, you are not allowed to upload this file type.

print_r($mimes) gives this (glb is included):

Array ( [jpg|jpeg|jpe] => image/jpeg [gif] => image/gif [png] => image/png [bmp] => image/bmp [tiff|tif] => image/tiff [webp] => image/webp [ico] => image/x-icon [heic] => image/heic [asf|asx] => video/x-ms-asf [wmv] => video/x-ms-wmv [wmx] => video/x-ms-wmx [wm] => video/x-ms-wm [avi] => video/avi [divx] => video/divx [flv] => video/x-flv [mov|qt] => video/quicktime [mpeg|mpg|mpe] => video/mpeg [mp4|m4v] => video/mp4 [ogv] => video/ogg [webm] => video/webm [mkv] => video/x-matroska [3gp|3gpp] => video/3gpp [3g2|3gp2] => video/3gpp2 [txt|asc|c|cc|h|srt] => text/plain [csv] => text/csv [tsv] => text/tab-separated-values [ics] => text/calendar [rtx] => text/richtext [css] => text/css [htm|html] => text/html [vtt] => text/vtt [dfxp] => application/ttaf+xml [mp3|m4a|m4b] => audio/mpeg [aac] => audio/aac [ra|ram] => audio/x-realaudio [wav] => audio/wav [ogg|oga] => audio/ogg [flac] => audio/flac [mid|midi] => audio/midi [wma] => audio/x-ms-wma [wax] => audio/x-ms-wax [mka] => audio/x-matroska [rtf] => application/rtf [js] => application/javascript [pdf] => application/pdf [class] => application/java [tar] => application/x-tar [zip] => application/zip [gz|gzip] => application/x-gzip [rar] => application/rar [7z] => application/x-7z-compressed [psd] => application/octet-stream [xcf] => application/octet-stream [doc] => application/msword [pot|pps|ppt] => application/vnd.ms-powerpoint [wri] => application/vnd.ms-write [xla|xls|xlt|xlw] => application/vnd.ms-excel [mdb] => application/vnd.ms-access [mpp] => application/vnd.ms-project [docx] => application/vnd.openxmlformats-officedocument.wordprocessingml.document [docm] => application/vnd.ms-word.document.macroEnabled.12 [dotx] => application/vnd.openxmlformats-officedocument.wordprocessingml.template [dotm] => application/vnd.ms-word.template.macroEnabled.12 [xlsx] => application/vnd.openxmlformats-officedocument.spreadsheetml.sheet [xlsm] => application/vnd.ms-excel.sheet.macroEnabled.12 [xlsb] => application/vnd.ms-excel.sheet.binary.macroEnabled.12 [xltx] => application/vnd.openxmlformats-officedocument.spreadsheetml.template [xltm] => application/vnd.ms-excel.template.macroEnabled.12 [xlam] => application/vnd.ms-excel.addin.macroEnabled.12 [pptx] => application/vnd.openxmlformats-officedocument.presentationml.presentation [pptm] => application/vnd.ms-powerpoint.presentation.macroEnabled.12 [ppsx] => application/vnd.openxmlformats-officedocument.presentationml.slideshow [ppsm] => application/vnd.ms-powerpoint.slideshow.macroEnabled.12 [potx] => application/vnd.openxmlformats-officedocument.presentationml.template [potm] => application/vnd.ms-powerpoint.template.macroEnabled.12 [ppam] => application/vnd.ms-powerpoint.addin.macroEnabled.12 [sldx] => application/vnd.openxmlformats-officedocument.presentationml.slide [sldm] => application/vnd.ms-powerpoint.slide.macroEnabled.12 [onetoc|onetoc2|onetmp|onepkg] => application/onenote [oxps] => application/oxps [xps] => application/vnd.ms-xpsdocument [odt] => application/vnd.oasis.opendocument.text [odp] => application/vnd.oasis.opendocument.presentation [ods] => application/vnd.oasis.opendocument.spreadsheet [odg] => application/vnd.oasis.opendocument.graphics [odc] => application/vnd.oasis.opendocument.chart [odb] => application/vnd.oasis.opendocument.database [odf] => application/vnd.oasis.opendocument.formula [wp|wpd] => application/wordperfect [key] => application/vnd.apple.keynote [numbers] => application/vnd.apple.numbers [pages] => application/vnd.apple.pages [glb] => model/gltf-binary )