Emacs

Is this only in Emacs 29 or have I really never known this existed for this long???

I think everyone has had that kind of experience :)

wow! thank you for that cool feature :) 8 years with Emacs and dont know about it :)

You can drag files into many different places and have the drop action do different things. The compose email interface will automatically attach the dragged files as attachments.

Another minor point: if you use VMWare workstation as I do, you can drag files from the host straight into an Emacs buffer. I'm sure this also works for other hypervisors like Virtualbox.

instill in me a strong sence that Emacs not only coexists within the operating system, but rather embodies the operating system itself.

It just implements being a drag-and-drop target; it does not "coexist within the operating system"

While it is true that Windows does not support dragging files from dired buffer to external locations, but I seldom encounter such a requirement in my day-to-day workflows.

~~Windows folders support being a drop-target; this is likely emacs not supporting /its/ side of the protocol.~~

Ah, I see the dired-mouse-drag-files note, below.

The mouse does seem to work in dired, it's just that it does emacs things, not OS things. :)

For example, on Windows, I can drag a file from the Windows Explorer and drop it into dired and it copies the file -- as OP observed. (Thanks for that, it's nice!)

But for the reverse, if I click on a file in dired and drag, it highlights a region, because click-and-drag in emacs does that. Likewise left-clicking on a file opens the file...in emacs, because that's what hitting RET would do when point is on the same line.

(There is a nice right-click context menu in dired, but it doesn't include anything for browse-url-of-dired-file -- bound to W -- which for the most part opens a file using the default Windows application.)

Set `dired-mouse-drag-files` to t, and you can also drag file from dired to other programs!