Message cannot be saved as draft (Yahoo attachments)

Hello,

I have done all this; I've even moved the file to be sure there's not an error on the disk preventing a full clean upload.  My intention is to attach a small (2.6Mb) ePub file to send it to be read on my Kindle device. I put the correct "To" email; I put a small default subject & body message, then click the paperclip (I'm on Windows 10 desktop, using an up-to-date Chrome, select the file, click "attach" & it seems to operate normally, but then a big red "!" appears instead of a document icon; eventually it goes away, but only after I get a popup error from Yahoo: Message cannot be saved as draft.

I click "send" & get error message in my inbox "your message could not be sent."

Anyone have an idea of what's going on?

Windows / Chrome 119.0.0.0