Fix #409

For this to work you will need to add in the main section of your config file:
VALID_UPLOAD_EXTENSIONS = fastq,fastq.gz,txt
You can make the config file's format slightly more robust by allowing spaces in the comma-separated list:
```python
self.valid_upload_extension = [x.strip() for x in config.get(
    'main', 'VALID_UPLOAD_EXTENSION').split(',')]
```