Lower extension size

[markberger]

In immutable/upload.py, there is the following comment:

# our current uri_extension is 846 bytes for small files, a few bytes
# more for larger ones (since the filesize is encoded in decimal in a
# few places). Ask for a little bit more just in case we need it. If
# the extension changes size, we can change EXTENSION_SIZE to
# allocate a more accurate amount of space.
EXTENSION_SIZE = 1000
# TODO: actual extensions are closer to 419 bytes, so we can probably lower
# this.

As part of #1382, I am removing the TODO comment and creating a ticket to take its place.