✨(backend) allow uploading more types of attachments

We want to allow users to upload files to a document, not just images.
We try to enforce coherence between the file extension and the real
mime type of its content. If a file is deemed unsafe, it is still accepted
during upload and the information is stored as metadata on the object
for display to readers.

src/backend/pyproject.toml

@@ -50,6 +50,7 @@ dependencies = [
     "PyJWT==2.9.0",
     "pypandoc==1.14",
     "python-frontmatter==1.1.0",
+    "python-magic==0.4.27",
     "requests==2.32.3",
     "sentry-sdk==2.16.0",
     "url-normalize==1.4.3",