check file upload threshold against all selected files

[j-ed]

Expected behavior

The configured file upload threshold should always be checked against all selected files and not only single files.

Current behavior

If you've set the upload limit to e.g. 2MB and select three files with a total size of 3MB the upload request will not be triggered.

Steps to reproduce

1. Open Thunderbird and create a new message
2. Select multiple files which together exceed the configured limit. -> No upload request is triggered.

Environment

Server Configuration

OS: Linux 3.16.47 Web server: Apache2 2.4.26 Database: MariaDB 10.0.32 PHP version: 5.6.30 Nextcloud version: 12.0.3

Client Configuration

Thunderbird version: 52.3.0 Nextcloud for Filelink version: 1.7 Operating system: Windows 7