diff --git a/slack_backup/objects.py b/slack_backup/objects.py
index caf316d..9b1beec 100644
--- a/slack_backup/objects.py
+++ b/slack_backup/objects.py
@@ -229,14 +229,14 @@ class Message(Base):
     channel = relationship("Channel", back_populates="messages")
 
     reactions = relationship("Reaction", back_populates="message")
-    files = relationship("File", back_populates="message")
+    file = relationship("File", uselist=False, back_populates="message")
     attachments = relationship("Attachment", back_populates="message")
 
     def __init__(self, data_dict=None):
         self.update(data_dict)
 
     def datetime(self):
-        return datetime.fromtimestamp(self.ts)
+        return datetime.fromtimestamp(float(self.ts))
 
     def update(self, data_dict):
         data_dict = data_dict or {}
@@ -256,7 +256,7 @@ class File(Base):
     filepath = Column(Text)
 
     message_id = Column(Integer, ForeignKey('messages.id'))
-    message = relationship('Message', back_populates='files')
+    message = relationship('Message', back_populates='file')
 
     def __init__(self, data_dict=None):
         self.update(data_dict)