In the "old times" Firefox asked me whether I wanted to download and then open, download into temp and then open, or download into a to-be-selected folder; with an addon I could force browse render as an option, too.
Yet, this required two mouse clicks in the "worst case" that the default selection didn't fit.
I think the best solution is: if no download attribute set and Content-Disposition is not "attachment", then render in the browser. If either condition is met, open a popup with large buttons immediately executing the action: download to default folder, download to temp (which will be deleted upon next browser start), download to specific folder, render in browser (if there's a registered handler plugin for this mimetype). Maybe add a checkbox "Open after download has finished", and maybe also add one "Try multi-stream download" (because many sites still limit single-stream bandwidth). Even more maybe, for video files add a "Preview with VLC" and fetch the required parts (header and final part of video) first, so I can stream and keep the file for later usage.
Yet, this required two mouse clicks in the "worst case" that the default selection didn't fit.
I think the best solution is: if no download attribute set and Content-Disposition is not "attachment", then render in the browser. If either condition is met, open a popup with large buttons immediately executing the action: download to default folder, download to temp (which will be deleted upon next browser start), download to specific folder, render in browser (if there's a registered handler plugin for this mimetype). Maybe add a checkbox "Open after download has finished", and maybe also add one "Try multi-stream download" (because many sites still limit single-stream bandwidth). Even more maybe, for video files add a "Preview with VLC" and fetch the required parts (header and final part of video) first, so I can stream and keep the file for later usage.