Hi there, @Electrician.

iCloud Document Sharing allows you to work with your QuickBooks Desktop for Mac 2020 from multiple devices connected to iCloud. 

Since you just upgraded to the latest version, let's make sure your company file is sync to an iCloud shared folder (Documents). Once done, open the file through the shared file. This way, you can access the updated file or attachments on all connected devices.

I’m adding steps on how to share the iCloud Files:

1. Go to the Finder icon, then find the file you want to share.
2. Select the company data to highlight it.
3. Click the Share button, then select Add People.
4. Choose how you want to send the invitation, for example, mail, messages, or social media.
5. Click Share.

However, if you're still getting the same result, I'd recommend consulting our QuickBooks Mac Care Team. There they can check your account and can help you verify what's causing this odd behavior when accessing attachments through your iPhone. 

Here's how:

1. Click Help at the top menu and select QuickBooks Desktop Help.
2. In the Have a Question? window, select Contact us.
3. Go to the first section and enter the details of your concern.
4. Click Search and move to the second section.
5. Select how you want to interact with them (phone or chat).

For additional reference, you can use the following article to know more about the features available in QuickBooks Desktop for Mac 2020: User Guide. 

Drop me a comment below if you have any other questions. Have a great weekend.