fix(docker): install python-magic and libmagic for upload MIME sniffing

Install libmagic1 and image-scoped python-magic in the Docker image so upload MIME detection can use content sniffing. Add regression coverage for the Dockerfile dependency pair and the libmagic-present sniffing path.

Dockerfile

@@ -32,6 +32,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libgl1 \
     libglib2.0-0t64 \
     libxcb1 \
+    libmagic1 \
     && rm -rf /var/lib/apt/lists/*
 
 # libgl1/libglib2.0-0t64/libxcb1 are runtime shared libs (libGL.so.1,
@@ -40,6 +41,14 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 # and dies with `libxcb.so.1: cannot open shared object file` despite a clean
 # pip install. Using full opencv-python (not -headless) because basicsr/gfpgan/
 # facexlib/realesrgan all depend on the `opencv-python` distribution by name.
+#
+# libmagic1 is the shared lib (libmagic.so.1) that python-magic dlopens for
+# content-based MIME sniffing in src/upload_handler.py. We install both here
+# (libmagic1 + the python-magic wrapper, below) rather than in requirements.txt
+# because python-magic resolves libmagic at import time: where the lib is
+# absent the import can block or raise, so keeping it image-only avoids
+# regressing pip/venv installs on hosts without libmagic. Debian always has the
+# lib here, so the import is instant and detection actually works.
 
 # Docker CLI (client only — daemon stays on the host via the
 # /var/run/docker.sock mount). The Debian `docker.io` package ships
@@ -67,6 +76,11 @@ COPY requirements.txt requirements-optional.txt ./
 RUN pip install --no-cache-dir -r requirements.txt \
     && if [ "$INSTALL_OPTIONAL" = "true" ]; then pip install --no-cache-dir -r requirements-optional.txt; fi
 
+# python-magic powers content-based MIME sniffing in src/upload_handler.py.
+# Image-only (not in requirements.txt) because it needs the libmagic1 system
+# lib installed above; see the apt note near the top of this stage.
+RUN pip install --no-cache-dir python-magic==0.4.27
+
 # Pre-install the patched basicsr/gfpgan/facexlib wheels built in the
 # realesrgan-wheels stage (--no-deps keeps the image lean — torch & friends are
 # pulled only when realesrgan is actually installed). With these dists already