feat: Use random filename suffixes for blobstorage (#4309)

Recently there was an accident with a chatbot that replaced its avatar set from the command line
with an unrelated avatar of a contact. Both the `selfavatar` setting and the contact avatar `i`
param pointed to `$BLOBDIR/avatar.png` at the time it was detected. How this happened is unclear,
but it is possible that `avatar.png` was removed, unmounted or otherwise not detected by the core,
and the core stored avatar received from the contact as `avatar.png`, while `selfavatar` config
still pointed to `$BLOBDIR/avatar.png`.

Such bugs are unavoidable even if the core itself has no bugs as we cannot rely on blobdir not
reside on the faulty network filesystem, being incorrectly backed up and restored etc., so we should
assume that files may be randomly removed. Then there may be dangling `$BLOBDIR/...` references in
the database which may accidentally point to unrelated files, could even be an `avatar.png` file
sent to the bot in private.

To prevent such bugs, we add random filename suffixes for the blobdir objects. Thanks to the added
Param::Filename these random suffixes aren't sent over the network.

python/tests/test_3_offline.py

@@ -440,31 +440,34 @@ class TestOfflineChat:
         assert msg.is_image()
         assert msg
         assert msg.id > 0
-        assert os.path.exists(msg.filename)
+        assert os.path.exists(msg.file_path)
         assert msg.filemime == "image/png"
 
     @pytest.mark.parametrize(
-        ("fn", "typein", "typeout"),
+        ("stem", "ext", "typein", "typeout"),
         [
-            ("r", None, "application/octet-stream"),
-            ("r.txt", None, "text/plain"),
-            ("r.txt", "text/plain", "text/plain"),
-            ("r.txt", "image/png", "image/png"),
+            ("r", "", None, "application/octet-stream"),
+            ("r", ".txt", None, "text/plain"),
+            ("r", ".txt", "text/plain", "text/plain"),
+            ("r", ".txt", "image/png", "image/png"),
         ],
     )
-    def test_message_file(self, chat1, data, lp, fn, typein, typeout):
+    def test_message_file(self, chat1, data, lp, stem, ext, typein, typeout):
         lp.sec("sending file")
+        fn = stem + ext
         fp = data.get_path(fn)
         msg = chat1.send_file(fp, typein)
         assert msg
         assert msg.id > 0
         assert msg.is_file()
-        assert os.path.exists(msg.filename)
-        assert msg.filename.endswith(msg.basename)
+        assert os.path.exists(msg.file_path)
+        assert msg.file_path.endswith(ext)
+        assert msg.filename == fn
         assert msg.filemime == typeout
         msg2 = chat1.send_file(fp, typein)
         assert msg2 != msg
-        assert msg2.filename != msg.filename
+        assert msg2.file_path != msg.file_path
+        assert msg2.filename == fn
 
     def test_create_contact(self, acfactory):
         ac1 = acfactory.get_pseudo_configured_account()
@@ -532,7 +535,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
     def test_import_export_on_encrypted_acct(self, acfactory, tmp_path):
         passphrase1 = "passphrase1"
@@ -570,7 +573,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
         ac2.shutdown()
 
@@ -587,7 +590,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
     def test_import_export_with_passphrase(self, acfactory, tmp_path):
         passphrase = "test_passphrase"
@@ -626,7 +629,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
     def test_import_encrypted_bak_into_encrypted_acct(self, acfactory, tmp_path):
         """
@@ -671,7 +674,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
         ac2.shutdown()
 
@@ -688,7 +691,7 @@ class TestOfflineChat:
         messages = chat2.get_messages()
         assert len(messages) == 2
         assert messages[0].text == "msg1"
-        assert os.path.exists(messages[1].filename)
+        assert os.path.exists(messages[1].file_path)
 
     def test_set_get_draft(self, chat1):
         msg = Message.new_empty(chat1.account, "text")