I miss this function too and would like to have it in the backend.
I am currently trying to upload a file from another JavaScript framework via /data/upload into R3 as described in https://community.rei3.de/d/456-datei-up-download-via-rest-api/8 . This works well with simple textfiles, but not with binary files. Creating a mulitpart/form-data request for uploading binary files with JavaScript seems to be an almost impossible task.
It would be helpful to scan directories and import files via a backend function and then link them to a data record using file_link. Not a perfect solution for my case, but at least a workaround.