Fix codename regex to match <p> instead of <span>
Update codename regex to handle all possible characters
Improve file field detection

Previous code was fragile and implicit. This code makes the failure case
explicit, based on the default value of {} passed to web.store as well as notes
from the Python CGI documentation, which is the source of the FieldStorage data
structure. [1]

[1] http://docs.python.org/2/library/cgi.html
unit test for file submission