Starting with eM Client 8 attachments can be uploaded to Dropbox, OneDrive and Google Drive and attached to your message as a link. OwnCloud and Nextcloud support was added in eM Client 8.2.

To use this feature, you need to have either of these apps installed on your device and login to both the app and login again via eM Client when you first attempt to use it. You can also set up the storage providers in Menu > Settings > Mail > Attachments.

When you attach a file from the Cloud storage folders you will get to decide if you want to insert the file or just the link.