✨(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.

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to
 
 ## Added
 
+- ✨(backend) allow uploading more types of attachments #309
 - ✨(backend) add name fields to the user synchronized with OIDC #301
 - ✨(ci) add security scan #291
 - ♻️(frontend) Add versions #277