double-escaping of file names when using the download button from the file tree

[bribroder]

We noticed that trying to download a file from a folder with a space in the name throws a 404--some users report this used to work and stopped recently. It appears that the folder names are being escaped twice, so

https://jupyter.com/user/bribroder/files/Folder Name/space test.csv?download=1

Is escaped to

https://jupyter.com/user/bribroder/files/Folder%2520Name/space%2520test.csv?download=1

%25 is the escaped version of %, and %20 is the escaped version of a space

The behavior only occurs from the tree view of a folder, by checking the box next to the file and clicking the download button:

opening the file in 'edit' mode and downloading from the File menu works correctly.

[bribroder]

Fixed by PR #5720