You have to manually go through android settings and change allow only media files to allow all files.
This worked for me with modified wifi app
Settings
Apps
Musicbee wifi sync
Permissions
Files and settings
I dont seem to be able to do this in on my samsung S20 Android 12, I cant seem to get it to have all files, media is the only one I got
Anyone else been able to get around wifi sync and needing access to the root sd card folder? It just stopped working probably a few months ago, there was no update to my phone in that time, Ive cleared all data from the app as well. I guess it could have been a google thing that was pushed through a play store update
Followed BoringName's steps, and it gave an access error-- continued to follow his steps by going to setttings (I also have a samsung, though its a 22 u) and searched for special access, selected the result, selected "all files access", and then found music bee wifi sync app and gave it access. Now it works 
My phone is not rooted, for context.
From reading your post, and what I did initially myself lol, I imagine you're checking permissions of the app specifically-- you can't get to where you need to be by looking at the specific app's access permissions. All file access is it's own menu found in settings (that lists all the apps that have requested the permission and shows if they have it or not), so you go there first and then find the app in question from that space ^_^
I tried that with no luck
From settings - apps it shows the app has files and media permission, click that it shows files and media, but if I click on files and media it shows allow access to media only. despite it saying files and media in the screen before. Go to the app, it stil wont let me select sd card
Sounds like you're still going to the app and then trying to find the setting. Don't do that, find the setting and then find the app.
If I understand correctly you're doing:
Search for app -> go into app's settings -> click file permissions -> see only media options -> no profit
Which to be fair makes the most sense, but just don't because it wont work lol (I did this first too).
You need to:
Search for "all file access" in the generic settings app (I don't know if you can even find it manually, so to save time I just use the search feature) -> select this, get into a menu with a big list titled special access -> select first option "all file access" -> find the app in question (music bee in this case) -> ensure the little flip switch is blue (toggled to the right, meaning active) -> profit