Replace stock file manager App with alternative for file upload in browser?

When I upload a file (e.g. into a post here) using the browser (Fennec F-Droid in my case), first there will be a choice of Apps displayed which can select a file, the choice being camera Apps and Files (the stock file manager App).

I can install and use other file managers without a problem (tested with Amaze, Dir, Ghost Commander and Total Commander), but they never show up as a choice for uploading a file in the browser.

Does anybody know why that is?

I looked into permissions but I saw nothing helpful there, and there’s no Standard App entry you could set for “choosing files” or similar.

Same here with different device (FP2), different OS (/e/) and different browsers (lightning, Firefox, /e/-Browser).
Just in LineageOS 17 it seems to work different: it seems to open the integrated file manager which then offers to search for files in other apps (as e.g. a different file manager). EDIT: depending on the Browser I may have to select the integrated file manager and then I get:


I get the same result you describe for LOS on my stock Android 10 FP3. Seems like this changed with the update from A9 -> A10.

The big question is what happens if you get rid of the integrated file manager… Experiences, anybody?