* Use ContentDisposition instead of ContentType to control download
Fixes#1197. Previously we were using a funky ContentType to attempt
to force a file download rather than display in browser, but this
conflicted with attempts to save UTF-8 which was outside the Basic
Multilingual Plane (BMP).
By switching to ContentDisposition: attachment, which has been
the preferred method for a number of years, we can avoid this conflict.
As part of this, switch to using the "preview" param consistently
to control preview vs download rather than the content type.
* Switch content type to text/plain
Now that we don't need to use ContentType to control download
behavior, we can use something more reasonable.
5f368bc56d