I added this to Port Request on GitHub, but I'm hoping people who want this will add to it, or perhaps someone who can port will have a look at it. 🙏
Backdrop already has IMCE ported. This add-on module allows linking to multiple existing files at once in the file field, not just through the WYSIWYG editor. (This functionality has been included in Drupal IMCE 8).
HOW IT WORKS
If you've got an events-orientated site with photo galleries, this is the module you need. I use it on every Drupal website. It allows you to link to 100 photos in a single action, PLUS keep your photo's tidy in subfolders.
IMCE File browser has lots of other useful functions for identifying and compressing large images.
1) I create a sub-folder and FTP all the photos. (That sounds like a hassle, but it's easier than fiddling with filefield paths, especially if you often delete old galleries.) You can upload files using the IMCE file browser interface if you're just doing a few.
2) On the file field, click "Open File Browser", highlight all the files and click Select.
3) All the files are attached.
I don't know what it does in Manage Files, but I hope it is the same as any other method.