Dotx uploading fails, even added to upload_mimes filter

[kisabelle]

function custom_upload_mimes ( $mime_types ) {
  $mime_types['dotx'] = 'application/vnd.openxmlformats-officedocument.wordprocessingml.template';
  return $mime_types;
}
add_filter('upload_mimes', 'custom_upload_mimes');

Even when added using the upload_mimes filter, MS Word Letterhead .dotx files fail to upload with the following error:

Sorry, this file type is not permitted for security reasons.

The only way to successfully upload is to allow unfiltered uploads.

[desrosj]

Hi @kisabelle,

Thanks for opening this! There is an issue tracking the overall fix to mime type related issues in #40175. This is most likely a side effect of r43988.

I will note this file type and ticket on #40175, but feel free to add any additional details you have to offer there!