$ pyflakes product/ERP5/Document/BigFile.py

    product/ERP5/Document/BigFile.py:27: 'getToolByName' imported but unused
    product/ERP5/Document/BigFile.py:180: undefined name 'DateTime'
    product/ERP5/Document/BigFile.py:325: local variable 'filename' is assigned to but never used
    product/ERP5/Document/BigFile.py:360: local variable 'data' is assigned to but never used

getToolByName is not used. For DateTime we add appropriate import. data
was there unused from the beginning - from 00f696ee (Allow to upload in
chunk.) - for query_range we just return range = [0, current_size-1] and
data is left unused.

I did not remove filename in

    # need to return it as attachment
    filename = self.getStandardFilename(format=format)
    RESPONSE.setHeader('Cache-Control', 'Private') # workaround for Internet Explorer's bug
    RESPONSE.setHeader('Accept-Ranges', 'bytes')

because as the comment says it tries to workaround some IE bug and I
have no clue is filename needed in that case and was forgotten to be
appended or it is the other way.
Reviewed-by: Romain Courteaud <romain@nexedi.com>