This allows download of attachments that are larger than current memory
limits, since we're not loading the entire file into memory any more.

For inline file responses, we take a 1kb portion of the file to sniff
before to check mime before we proceed.

Fixes hitting memory limits where downloaded file sizes are much greater
than memory limit. Stopping and flushing output buffer seemed to stop
limits causing issues when fpassthru is used.
Tested with 24M memory limit and 734M file

Required some special handling due to the content being base64-encoded
within a JSON response.

- Added testing check to buffer stop/clear on streaming output due to
  interference during tests.
- Made content-disposition header a little safer in download responses.
- Also aligned how we check for testing environment.