I've just had this too. I drafted an email in Thunderbird, edited the draft on my Android phone, and then updated the draft on Thunderbird again and this attachment was then present.
I've repreated this action and it does seem that the act of editing the draft on the phone is causing the attachment to appear. If I just save the draft, close it and reopen it on Thunderbird on my PC, the attachment does not appear.
The file in my case was called "nsmail.enpasscard".
In Thunderbird it appears in the attachment box as "Attached Message Part", with the enpass logo preceding it. In my case, hovering over the attachment showed the path "file:///C:/Users/Rob/AppData/Local/Temp/nsmail.enpasscard"
I opened this with notepad++ and the contents appear to be the email draft itself.
The mail app on my phone is the standard email app on my Galaxy S8, v6.1.01.0. Enpass version on Android is 126.96.36.1998