FileField fails validation when empty (Flask-WTF 0.14)

error.py

# Work successfully in my tests
resp = self.client.post(
    url_for('posts.post'),
    data={
        'body': 'Test',
        'upload': '',
    },
    follow_redirects=True
)

# However if I upload the code using multipart-form-data (taken from Firefox inspector `Params`, same issue in Chrome)
"""
Content-Type: multipart/form-data; boundary=---------------------------191598866112415400995302957
Content-Length: 651

-----------------------------191598866112415400995302957
Content-Disposition: form-data; name="body"

Test
-----------------------------191598866112415400995302957
Content-Disposition: form-data; name="csrf_token"

IjY4YTZmYjNmNWM4MWJlNmVjMjc3Y2Y4YjBiMTM1Nzk3YTdhMGZkNjci.C1VoNg.8Pg2PvN8N00Phc527XVBXjAn9rs
-----------------------------191598866112415400995302957
Content-Disposition: form-data; name="permission"

0
-----------------------------191598866112415400995302957
Content-Disposition: form-data; name="upload"; filename=""
Content-Type: application/octet-stream


-----------------------------191598866112415400995302957--
"""

# This has only started happening since since Flask 0.14 upgrade