We use Citrix ShareFile for secure file uploads.
Per their website: " The ShareFile USA Software-as-a-Service (SaaS) application is hosted in SSAE 16 Type II (SOC 1) audited datacenters, while the ShareFile EU SaaS is hosted in ISO 27001 certified datacenters. All datacenters holding customer files are SSAE 16 Type II (SOC 1)/SOC 2 audited and ISO 27001 certified. Stored files are protected using AES 256-bit encryption (a Federal Information Processing Standards (FIPS) approved encryption algorithm) in addition to unique per-file keys."
You can read more about their security measures here: https://www.sharefile.com/resources/citrix-sharefile-security-and-compliance-frequently-asked-questions.