Files from OneDrive are attached in emails as cloud attachments by default
When you attach a file to an email from OneDrive for Business, you may notice that the file is attached as a cloud attachment by default and not a local copy.
The setting for OneDrive files to be shared as cloud attachments by default has now been rolled back. In order for the change to take effect, please restart Outlook 2016, and you should go back to the original experience of seeing both a local copy of your OneDrive file in the Attachments menu, in addition to the cloud link, allowing you to select how you prefer sending your OneDrive file.
OneDrive cloud attachments are there to promote collaboration by allowing you to share a single version of the document, automatically permissioned to your recipients without going to OneDrive. In general, if you want to send a copy of the cloud file instead, you can select Attach as copy from the attachment menu. This option has always been available to you to choose.
Another Option:
Allow users to change the Outlook 2016 attach from one drive behavior. – Customer Feedback for Microsoft Office 365
start a new email. click file, quick access toolbar, add 'attach file' to the quick access toolbar. Essentially you have 2 attach file icons now, the one at the very top left of the window is the one that will allow you to insert classic attachments
Allow users to change the Outlook 2016 attach from one drive behavior. – Customer Feedback for Microsoft Office 365
The "Attach File" button from Outlook 2013 is also present in the new Outlook 2016, although not present by default in the Default Ribbons. You can add the "Attach File" button of Outlook 2013 by customizing your ribbon.
To do this, Click on New Email to show the compose email window. Right click on the upper tab where the ribbon is present and click "Customize the Ribbon.." button. Under the New Mail Message tab, create a custom ribbon group by clicking on the "New Group" button. After creating the new custom group, you can now add custom commands to that custom group.
On the left side of the panel, select the drop down menu and select All Commands. Find the command that says "Attach File" (the one without the arrow beside it.), select the command, and press add. After doing so, you can use that command to always upload attachments as a copy instead of choosing the attach as a link.