Drag and drop from Photos library to the Mac

As far as I know, dragging and dropping from Photos will save the current preview image.

This is usually a JPEG if the image has adjustments applied or is larger than the typical preview. Then Photos will render a smaller JPEG preview.

But if the original image file has no adjustments applied and is small enough to be used as a preview and is already a JPEG, the original will be dragged to the desktop.

Definitely.

I am usually using the File > Export command to export files from photos. Only then I can specify exactly, how the file should be rendered.

It has been the same in Aperture and iPhoto, Tom. You may be right, that it is a legacy option. With Apple pushing us towards using the apps full-screen, there is no longer space left for dragging and dropping items, we have to copy and paste.

If the original is very small, it might be dragged in the original format. I just dragged a small PNG screenshot from Photos and it has been saved as PNG.

I am not dextrous enough to do that successfully on my iPad, when the train is bumping up and down on the rails. 😊

On my Mac I had sometimes dangerous effects, when dragging photos directly to a different app, for example from Photos to Photoshop (on High Sierra). For small images, where Photos used the original as the preview, the original image file inside the library package got modified by Photoshop.