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:

• How to convert a Quote to an Invoice [Video].
• Copy an estimate or quote to a purchase order.
• Create partial invoices from an quote.

I'm always around if you have other concerns in sending your Quote.