Q: How to send large File attachment using iCloud mail

Are you signed into your iCloud account, when you try t send the attachment? Then tit should be automatic:

Send large attachments using Mail Drop

You can use Mail Drop to send files that exceed the maximum size allowed by your email account provider. Mail Drop uploads large attachments to iCloud, where they’re encrypted and stored for up to 30 days.

• If you have an iCloud account and you’re signed in to iCloud when you click Send, Mail automatically sends the attachments using Mail Drop. Mail Drop attachments don’t count against your iCloud storage.
• If you don’t have an iCloud account, or if you’re not signed in, Mail asks you whether to use Mail Drop (to always use Mail Drop select “Don’t ask again for this account”).

If a recipient uses Mail in OS X Yosemite or later, the attachments are included in your message. For other recipients, your message includes links for downloading the attachments, and their expiration date.

You can turn Mail Drop on or off for an account. Choose Mail > Preferences, click Accounts, select the account, click Advanced, then select or deselect “Send large attachments with Mail Drop.”

No, the help text i quoted is from the Mac Mail application. Mail.app on your Mac should automatically send your large attachment using Mail drop, if you are signed into iCloud account in your System preferences iCloud.

Wich Mac OS X version do you have?  My iCloud Mail account looks different in the Mail Preferences:

It is showing the option "Send large attachments with Mail drop":

I am using MacOS X 10.11.4.

That is the problem  MailDrop requires Yosemite at least:

see this link:  Mail Drop limits - Apple Support

Ok. Now I need to determine which OS on this Macbook.  Thank you.

Click the  icon in the main menu bar at the top of your display, and select  > About this Mac.  The about panel will show the system version.

It appears Apple is pushing El Capitan, but there are some pretty negative reviews.

It is more important to check if El Capitan supports the applications you are using and your hardware is wrking, scanners, printers, audio interfaces, etc. I would hasitate to upgrade, if it meant to upgrade expensive applications as well.

El Capitan added security features, that some users do not like. It is no longer possible to modify the system applications, for example to modify the icon of your mail application. This is to prevent malware from installing itself in your applications folder.

I find El Capitan faster and more responsive than Yosemite.