Short answer? Very secure.
Longer answer? Very, very secure.
Longer-longer answer? Simply put; we need to stick to the highest standards possible because we're located in the European Union, and are therefore governed by the EU's data protection laws.
Our privacy and security standards are compliant with the high level of personal data protection required by GDPR and the Dutch UAVG (Uitvoeringswet Algemene Verordening Gegevensbescherming).
Your files are encrypted when they are being transferred (TLS) and when they are stored (AES-256). Once your files are safely stored, they can only be accessed using the unique links sent to the sender and recipient.