get_params method should seek at 0 on the file stream

[chfw]

continuing with #26, explicit seeking at 0 on the file stream could help avoid #37.

def get_params(self, **keywords):
    extension = self.name.split(".")[-1]
    keywords['file_type'] = extension
    self.file.seek(0) # <--
    content = self.file.read()
    if content:
        keywords['file_content'] = content
    else:
        raise IOError("No content was uploaded.")
    return keywords