If you are trying to upload an attachment to a slip and are receiving an error, this may be down to the filename. Do you have any errant full stops in the file name? The system reads anything after the full stop as the file type, and this is the most common cause. So if the attachment filename was:
This would be fine, as it is a recognised file extension. However, if it was:
This may throw an error, as everything past the first full stop would be read as the file type. This would then not be recognised. To resolve this, you would just need to remove the full stops from the date format and try uploading again.