Sending Folder IDs instead of Folder path in APIs

Yes, you can upload a file into a folder, using a path relative to the ID for the folder, like "<FOLDER ID>/<FILE NAME>".

Here's a simple example of what that would look like:

curl -X POST https://api.dropboxapi.com/2/files/create_folder_v2 \
    --header "Authorization: Bearer <ACCESS_TOKEN>" \
    --header "Content-Type: application/json" \
    --data "{\"path\": \"/test_256136_folder\"}"

# {
#   "metadata": {
#     "name": "test_256136_folder",
#     "path_lower": "/test_256136_folder",
#     "path_display": "/test_256136_folder",
#     "id": "id:25N5ksooX-sAAAAAAAMgxg"
#   }
# }

curl -X POST https://content.dropboxapi.com/2/files/upload \
    --header "Authorization: Bearer <ACCESS_TOKEN>" \
    --header "Dropbox-API-Arg: {\"path\": \"id:25N5ksooX-sAAAAAAAMgxg/test_256136_file.txt\"}" \
    --header "Content-Type: application/octet-stream" \
    --data-binary "test data"

# {
#   "name": "test_256136_file.txt",
#   "path_lower": "/test_256136_folder/test_256136_file.txt",
#   "path_display": "/test_256136_folder/test_256136_file.txt",
#   "id": "id:25N5ksooX-sAAAAAAAMgxw",
#   "client_modified": "2017-12-11T15:45:12Z",
#   "server_modified": "2017-12-11T15:45:13Z",
#   "rev": "72689021eccc7",
#   "size": 9,
#   "content_hash": "824979ede959fefe53082bc14502f8bf041d53997ffb65cbbe3ade5803f7fb76"
# }