Drag'n'Drop file in SSMS = SQL Assistant disabled

There could be a few factors affecting that, including

1. Elevation level, file recipient needs to have the same or higher elevation
2. UAC virtualization, hard to say exactly where it impacts, sometimes it impacts in very unexpected ways.
3. 32-bit vs 64-bit, that needs to be the same on both ends, from/to programs

All this can be checked with the Task Manager. On the Details tab if you right-click column headers, you can use Select Columns menu to add additional columns to the view. Add the above and check if Total Commander , Windows Explorer, and SQL Editor show the same characteristics

I'd rule out #1. In case the recipient is of a lower elevation level, the os refuses the drop operation. I noticed this a few times when my file manager was run as administrator and SQL Editor was not. SQL Editor simply did not open the file as if nothing has happened. But your file manager might behave differently, so it's definitely worth checking.

One more thing I'd like to add too. In the past I've noticed that sometimes dropping files in the SQL Editor workspace doesn't work, while dropping them on the toolbar does work.