[feature request] display download size

[matteomazza91]

UI request

in the window listing all files being available to download, it would be nice to have

• the size of the files being transferred (next to each file)
• the total size (sum of previous values)
• the size of the zipped archive.

HTTP header request

• add the size of the download so that the browser (or client in general) can compute estimation

details

I know that the zip archive is created on the fly and so it is impossible to know the size before, but maybe is it a possible solution to spawn a thread that create the zip on the fly (without storing it) just to compute the size?