✨(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.

src/backend/impress/settings.py

@@ -212,7 +212,11 @@ class Base(Configuration):
         "application/x-msdownload",
         "application/xml",
     ]
-
+    DOCUMENT_ATTACHMENT_CHECK_UNSAFE_MIME_TYPES_ENABLED = values.BooleanValue(
+        True,
+        environ_name="DOCUMENT_ATTACHMENT_CHECK_UNSAFE_MIME_TYPES_ENABLED",
+        environ_prefix=None,
+    )
     # Document versions
     DOCUMENT_VERSIONS_PAGE_SIZE = 50