R1:d6928a3c2679

When creating a File storage object for a Paste, try to give it the same name as the Paste

Summary:
Ref T13528. Paste data is stored in files, but the files are always named "raw.txt".

Now that Paste provides a hint to use Files for "DocumentEngine" rendering, try to use the same name as the paste instead.

Test Plan:
- Created a paste named "staggering-insight.ipynb".
- Clicked "View as Jupyter Notebook" from Paste.
- Saw a file named "staggering-insight.ipynb", not "raw.txt".
- Created a paste…