This code snippet extends the "File upload" field type to allow the vCard (also known as VCF) file types.

um_add_vcard_file_types.php

<?php

/**
 * Allow to upload vCard files via the "File Upload" field type.
 * Add this code to the file functions.php in the active theme directory.
 */
 
function um_add_vcard_file_types( $allowed_types ) {
	if( is_array( $allowed_types ) ) {
		$vcard_types = array(
			'vcf'   => 'vcf - vCard',
			'vcard' => 'vcard - vCard',
		);
		$allowed_types = array_merge( $vcard_types, $allowed_types );
	}
	return $allowed_types;
}
add_filter( 'um_allowed_file_types', 'um_add_vcard_file_types' );

function um_add_vcard_check_filetype_and_ext( $wp_filetype, $file, $filename, $mimes ){
	if ( empty( $wp_filetype['type'] ) && preg_match( '!\.(vcf|vcard)$!i', $filename ) ) {
		$mimes['vcf|vcard']  = 'text/vcard';
		$filetype            = wp_check_filetype( $filename, $mimes );
		$wp_filetype['ext']  = $filetype['ext'];
		$wp_filetype['type'] = $filetype['type'];
	}
	return $wp_filetype;
}
add_filter( 'wp_check_filetype_and_ext', 'um_add_vcard_check_filetype_and_ext', 10, 4 );

This gist is a part of the article um_allowed_file_types.

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/