Add asAttachment argument to answerFile proc
**Is your feature request related to a problem? Please describe.** 🤔
I want to be able to download the file sent using answerFile instead of displaying it inline, and to do that you can add `("Content-Disposition", "attachment")` to the http headers, but answerFile does not expose its HttpHeaders

**Describe the solution you'd like** 💡
allow some way of allowing files to be sent as attachments instead of inline

**Describe alternatives you've considered** 💁‍♀️
Could alternatively allow passing an HttpHeaders argument like answer accepts and then append the content type if it doesn't exist or something.
