Curious head-scratching case of Mail not sending attachments when signature is on

TL;DR: Every time i use Mail to send a Google email to an email address that's not with Google, 1) they don't see my signature properly and 2) they don't get my attachments.

I had to run a lot of tests to narrow down the problem. It all started when I kept sending emails with attachments from my Google account using Mail, but the recipients kept insisting I hadn't attached anything. Thought they were insane! But then I did a test myself and they were right. Sent the same email to my Hotmail account and I couldn't see any attachments, including the picture in my signature.

So I ran a bunch of tests by sending the same email to myself in different ways to different addresses and opening them up using my web browser and Mac Mail, and basically:

* No matter how i send them, if i open the email on Mac Mail, I cannot see the picture in my signature. It just doesn't open (btw, the picture is technically inserted from a file in my Google Workspace drive). I also don't see them if i open the emails on Hotmail.com.

* On Mac Mail and Gmail.com through the browser, I can always see the files I attached.

* But when I go to hotmail.com, I don't see the attached files if I have my signature inserted.

* If I send the same emails using Gmail.com on my internet browser, no issues, whether I have a signature or not. Recipients (me, in this case, using hotmail or google) always get the email, showing the attachments and the picture in my signature.

Go figure!

How do I fix this?