I often find myself writing up an email with the file I intend to attach showing in an already open Finder window, but buried beneath a gazillion other windows. In the past I found the fastest/simplest way to get that file attached to the email was to simply switch to Finder, select and start dragging the file, launch Exposé, keep on dragging until I’m hovering over the email window, hit space to select the email window, and the let go. That would attach the file to the email, I’d send it, and all would be done in just a second or two.
I don’t know exactly when this started, but it’s definitely been going on in High Sierra that I will often have the file not attach to the email. The whole Exposé part will work just fine including selecting and then dropping onto the email window (I even see the cursor show up indicating where the file will be inserted in the email), but then once I let go of the mouse button, the file doesn’t actually get pasted into the email. Essentially nothing happens. I usually can get it to work if I try it a second or third time. Any ideas what could be going wrong here? This always used to work really well (never had to try a second time) and AFAIK there’s nothing installed that would alter the Finder’s or Mail’s default behavior, except maybe Dropbox’ Finder integration that I would think is not related to Exposé.
I can get it to work by using Copy in Finder and then Paste in Mail, but that seems a bit Windows-y. Drag & drop has been such a natural method in macOS and usually works just fine between Mail and Finder (eg. dragging a file onto Mail will trigger a new email with that attachment).