Hi @joymunim,
The warnings in the log are just warnings, and can be ignored. We’ll be gradually working to resolve them over the next few releases. They aren’t related to the problem you’re having uploading images, stylesheets, etc.
There is a FAQ entry describing the likely causes and solutions of this error message. See Error message on upload: "No file uploaded or invalid file type!".
Regards,
Alec Smecher
Public Knowledge Project Team