Issue #1772: let the user choose uploads_in_blob with mongodb

About this issue:
https://code.google.com/p/web2py/issues/detail?id=1772

and because of @mdipierro commit https://github.com/web2py/web2py/commit/97739e9e8a9cac642c47f1d7e0adbc9cc5ec4cc8

this give users the possibility to choose, also doesn't break compatibility to apps using blob storage in web2py 2.7.4 and above.