✨(back) allow to disable checking unsafe mimetype on attachment upload

We added the possibility to scan all uploaded files with an anti malware
solution. Depending the backend used, we want to give the possibility to
check the file mimtype to determine if this one is tagged as unsafe or
not. To this you can set the environment variable
DOCUMENT_ATTACHMENT_CHECK_UNSAFE_MIME_TYPES_ENABLED to False. The
default value is True.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to
 - 📝(project) add system-requirement doc #1066
 - 🔧(front) configure x-frame-options to DENY in nginx conf #1084
 -  (doc) add documentation to install with compose #855
+- ✨(backend) allow to disable checking unsafe mimetype on attachment upload
 
 ### Changed