Uploads from Android app considered a subdirectory by `nosub`

[Emptiest]

Describe the bug

The issue here is kind of two-fold.

1. Uploads from the Android app always upload to / with no option to choose a different volume or subdirectory.
2. Uploads from the Android app are considered subdirectories by copyparty, which means if you have nosub enabled on the / volume, copyparty will block the upload with a message like http500: no subdirectories allowed, '/image-34.png'

The Android app is from the readme: https://f-droid.org/packages/me.ocv.partyup/

To Reproduce

1. Install the Android app
2. Configure it for a Copyparty server
3. Configure the Copyparty server with nosub enabled on /
4. Attempt to upload a file from Android using the app

Additionally, removing nosub from the volume and restarting copyparty makes the app work again.

Expected behavior

The upload should just work.

Server details (if you're NOT using docker/podman)

remove the ones that are not relevant:

• server OS / version: Windows 10
• what copyparty did you grab: (sfx/exe/pip/arch/...) exe
• how you're running it: (in a terminal, as a systemd-service, ...) In a terminal
• run copyparty with --version and grab the last 3 lines (they start with copyparty, CPython, sqlite) and paste them below this line:

copyparty v1.18.8 "logtail" (2025-07-31)
  CPython v3.13.5 on Windows64 10.0.19045 [MSC v.1943 64 bit (AMD64)]
   sqlite 3.49.1*1 | jinja 3.1.6 | pyftpd 1.5.10 | tftp 0.4.0

• copyparty arguments and/or config-file:

[/]
  E:\Share\Temp
  accs:
    rw: *
  flags:
    sz: 1k-100m           # accept upload only if filesize in this range
    df: 50g             # free disk space cannot go lower than this
    vmaxb: 1g           # volume can never exceed 1 GiB
    vmaxn: 4k           # ...or 4000 files, whichever comes first
    nosub               # must upload to toplevel folder
    lifetime: 300       # uploads are deleted after 5min
    maxn: 250,3600      # each IP can upload 250 files in 1 hour
    maxb: 1g,300        # each IP can upload 1 GiB over 5 minutes

Client details

if the issue is possibly on the client-side, then mention some of the following:

• the device type and model: Google Pixel 9a
• OS version: Android 16
• browser version: f-droid app https://f-droid.org/packages/me.ocv.partyup/