Python 3.x and empty file (b'') using FieldStorageUploadConverter

[antoniojesvs]

Using Pyramid and Python 3.4.
When I leave an input type="file" name="file" empty, the request.POST has {'file': b' '}.
b' ' is iterable, so formencode raises singleValueExpected=_('Please provide only one value')

Python 3.4.3 (default, Jul 28 2015, 18:20:59) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from formencode import Schema, validators
>>> class FileSchema(Schema):
...     file = validators.FieldStorageUploadConverter()
... 
>>> FileSchema().to_python({"file": b''})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/aj/.virtualenvs/marketplace/lib/python3.4/site-packages/formencode/api.py", line 476, in to_python
    value = tp(value, state)
  File "/home/aj/.virtualenvs/marketplace/lib/python3.4/site-packages/formencode/schema.py", line 217, in _convert_to_python
    value_dict, state, error_dict=errors)
formencode.api.Invalid: file: Please provide only one value
>>>

[lambacck]

We are explicitly checking for basestring, which seems like it's not the right thing to do on Python 3. I'm investigating what other string-likes we need in order to make it work.

[jvanasco]

this got detected by one of my test suites today. it was a pain to figure out. the fix deployed by @valos above passed out tests without triggering new issues.