Enable user save file with original name

Add file's original name in the header, ref to the RFC 2231
Merge pull request #7 from yangh/master

Enable user to save file with original name
revert PR #7 - the actual behavior is ok
Merge branch 'master' into tomgross-ptc

Conflicts:
	src/plone/app/blob/field.py
	src/plone/app/blob/interfaces.py
pep8ify