Add more mimes to support system. When nothing else works we can use map_meta_cap

support-system-add-mimes.php

add_filter( 'map_meta_cap', function( $caps, $cap, $user_id, $args ){

	if( ! isset( $_FILES['support-attachment'] ) ){
		return $caps;
	}

	$allowed_mimes = array(
		'jpg' =>'image/jpg',
		'jpeg' =>'image/jpeg',
		'gif' => 'image/gif',
		'png' => 'image/png',
		'zip' => 'application/zip',
		'gz|gzip' => 'application/x-gzip',
		'rar' => 'application/rar',
		'pdf' => 'application/pdf',
		'txt' => 'text/plain',
		'doc' => 'application/msword',
		'docx' => 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
		'ppt' => 'application/vnd.ms-powerpoint',
		'pptx' => 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
	);

	$allowed_extensions = array( 'jpg', 'jpeg', 'gif', 'png', 'zip', 'gz|gzip', 'rar', 'pdf', 'txt', 'doc', 'docx', 'ppt', 'pptx' );

	$filename = $_FILES['support-attachment']['name'][0];
	$filetype = $_FILES['support-attachment']['type'][0];
	$ext = pathinfo( $filename, PATHINFO_EXTENSION );	

	if( ! in_array( $ext, $allowed_extensions ) || ! isset( $allowed_mimes[ $ext ] ) || $allowed_mimes[ $ext ] != $filetype ) {
	    return $caps;
	}

	if ( $cap == 'unfiltered_upload' ) {
//If we use membership we can limit cap to members
		$args = array(
            		'user_id' => $user_id,
        	);

    		$args['meta_query']['status'] = array(
                        'key' => 'status',
                        'value' => array( 'trial_expired', 'expired' ),
                        'compare' => 'NOT IN',
                );    

    		$user_subscriptions = MS_Model_Relationship::get_subscriptions( $args );

    		if( ! empty( $user_subscriptions ) ){
			$caps = array();
			$caps[] = $cap;
		}
	}

	return $caps;

}, 99999, 4 );

add_filter( 'incsub_support_allowed_mime_types', function( $mime_types ){

	$mime_types[ 'doc' ] = 'application/msword';
	$mime_types[ 'docx' ] = 'application/vnd.openxmlformats-officedocument.wordprocessingml.document';
	$mime_types[ 'ppt' ] = 'application/vnd.ms-powerpoint';
	$mime_types[ 'pptx' ] = 'application/vnd.openxmlformats-officedocument.presentationml.presentation';

	return $mime_types;

}, 99999, 1 );