Why is QBO refusing to attach a .msg to an estimate?

Hello there, @Electrogas. I'm here to share why you can't attach a .msg file to your estimates in QuickBooks Online (QBO).

QBO has certain limitations on the file formats used within the program. This is done to safeguard the data's integrity and prevent malicious file uploads that could potentially harm your account. I suggest using only these formats or converting your files to any of them before attaching them. The supported attachment file types include PDF, JPEG, PNG, DOC, XLSX, CSV, TIFF, GIF, and XML.

I'm adding this article for detailed information: Add or delete attachments in QuickBooks Online.

Additionally, you can check this article on how to personalize your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please let me know if you have further questions about file formats in QBO. I'm always happy to help. Have a great day!

That completely failed to answer my actual question and just reiterated information that I already knew.

At least half of those file formats are more likely to be harmful than a .msg file, so I really don't believe that the restriction is for security reasons, but is more likely to just be another basic QOL feature that Intuit isn't willing to pay a dev to implement.  Honestly, given how many basic, obvious feature requests have literally years of forum posts asking for, I have to wonder if Intuit even has any devs working on QBO at all.