It has been detected in international sites, particularly outside of the US, that the WP function that produces a listing of acceptable file types used by Custom Form/CRM can produce faulty results.  If using the KK Upload field, it may be necessary OR suggested that the following WP filter be used to generate a file type list of your choosing.  Please note that specific Microsoft file types/extensions (.doc, .docx, .xlsx, etc.) may not be assigned or tagged by certain browsers with the proper mime if at all.  This module accounts for the first three MS file extensions shown with internal code:

//$mimes is an array in the form of $mimes[$file_extension] => $mime

function my_mime_list( $mimes ) {

$options = array();

$options[‘pdf’] = ‘application/pdf’;

return $options;


add_filter( ‘kk_custform_mime’, ‘my_mime_list’ );

The function above would return a single entry in the options for file type selection. Add as many as needed. If you want to preserve the mime list as it is given, then make $options = $mimes and proceed to cull what is not needed.