Start with the decision
Before choosing a transfer tool, decide what is actually at risk: the file contents, the recipient list, the link, the filename, the retention window, or proof that the file was opened.
Do not stop at HTTPS
HTTPS matters, but it only protects the connection. For confidential files, also ask what happens before upload, what is stored, who holds the key, and whether the provider can read the file later.
Make access boring
Good file delivery should make the access decision obvious: this person can open this package until this date. Anything broader becomes harder to explain and harder to clean up.
Read the security claims literally
Skip slogans and look for specifics: where encryption happens, whether keys reach the server, what metadata remains, whether links can be forwarded, and what an expired transfer really means.