Modifying files in Google Cloud Storage from Google App Engine
You can store the file in the tmpfs partition that you have on App Engine mounted in /tmp. It's in memory file system and you will use memory to store files. If the files are too large, increase the memory size of your App Engine instance else you will have a out of memory error
If the file is too big, you have to use another product.
Think to clean the files after use to free memory space.