Sometimes you get `Content-Type: application/x-gzip` instead of a chart

[grzkv]

Here's a weird behavior that @grzkv was able to reproduce:

• when I request the linear regression of hourly 75%iles of actions from my service, it sends me response with Content-Type: application/x-gzip and my browser tries to download the response body to disk
• if i change 75 to 99 (to get 99%ile) but otherwise the same expression, it sends me Content-Type: image/png

Expected behavior: the service should behave consistently. In this case, both should render a chart in my browser (png or otherwise).

[faceair]

Are you using Firefox? May be related to browser compatibility, chrome is ok.