Why do attachments get removed from emails I forward?
Apparently in a recent Apple Mail update the "forward attachments" behavior had the default changed. After many years of being able to forward an attached file with a simple "Forward" command I found that not only did the forwarded message not contain the attachment, the Apple Mail application also would not allow a Drag-n-Drop of the attachment from the received message to the new outgoing message. Clearly this was different behavior than I had previously gotten from the Apple Mail client.
(There are earlier threads on this subject - this answer applies to OS 13.1 Ventura Feb 2023)
Buried in the EDIT menu in Mail is a setting for INCLUDE ATTACHMENTS WITH REPLIES -> If that setting was there in earlier versions I know I never had to invoke it. But clicking the ASK choice worked for me.
MacBook Pro Apple Silicon 2023