Hello there, jr-justlec.
QuickBooks will send your attachment randomly. With that said, I'll share this good idea to our developer. So the you'll have the option to sort the added attachment.
As a workaround, you can add a number on the file name. By the time you'll send the attachment, your customers will have a clue on the sequence of your added files.
Additionally, you can also convert your Quote to an invoice or purchase order. Then, you can do a progress invoicing with quotes. You can check out this article as your reference:
I'm always around if you have other concerns in sending your Quote.