How do I attach a file to an email?
When you compose a email in a thread, use the ‘Attach a file’ function which comes with the message preview—it lists the Slack files you have access to. If your file isn't in Slack yet, upload it first to a channel or DM (files cannot uploaded in Slack threads).
When you compose an email in a channel (e.g. email group or subchannel), use Slack’s Share ability to bring an existing Slack file to the channel, or upload a new file (drag and drop the file or click on the + to the left of the input box). Either way, MailClark knows to attach the file in your email.