Flask, Apache, mod_wsgi: unable to save file on server side
Youre using /uploads as your path.That means you're trying to upload to a directory named /uploads at root level of your filesystem.
This is usually wrong and normally it's the error.
If you've the uploads folder under your flask application file structure, then you should create the path using app.root_path which holds the absolute application path.
Something like
file.save(os.path.join(app.root_path, '/uploads', filename))