Feature proposal: more dynamic HEAD checks.

[benbart]

This is an idea I had earlier today. It's not specifically needed by ASF, but it may be feature that would help another user.

TEA currently checks if an object exists in S3 before creating a signed URL. This takes a little time, so we provide the option to turn off that check. This is a binary choice, either all or none are checked, depending on that setting.
https://github.com/asfadmin/thin-egress-app/blob/devel/lambda/app.py#L294-L301

Would it be useful to do checks for some types of files and not others?