You can attach files and photos from your mac or PC using the free WePrint "Send to Device" feature. Firstly, install WePrint on your Mac or PC from here:
https://esz.nz/wp

Create a new email on your iPhone/iPad and leave it open.
Open the WePrint window and tap on the "Transfer" tab. You should now see your iPhone or iPad in the list of devices. You can now drag files and photos onto the device icon inside the WePrint window and the files will be transferred to your iPhone/iPad and attached the email that is open.

On a Mac, you can also print documents to send to AltaMail as a PDF. Open a document print dialog. Tap to add a new printer and select "Send to Device". Then when you print the document, WePrint will open and allow you to send the new PDF to your iPhone/iPad send email screen.

If your iPhone/iPad is not available inside WePrint, make sure your device and the WePrint computer are in the same network. Also make sure you have the "WiFi clipboard transfer" turned on in the AltaMail settings - Clipboard section.