**Steps to replicate the issue** (include links if applicable):
* Take an m3u8 file like: https://upload.wikimedia.org/wikipedia/commons/transcoded/1/10/Bombay_Cat_kitten_video_of_Blue_Sinfonie.ogv/Bombay_Cat_kitten_video_of_Blue_Sinfonie.ogv.m3u8
* Inspect the request
*
**What happens?**:
Response header content-type reports `text/plain`
**What should have happened instead?**:
content-type should be `application/x-mpegURL` or perhaps `application/x-mpegURL; charset=utf-8` ?
**Software version** (skip for WMF-hosted wikis like Wikipedia):
**Other information** (browser name/version, screenshots, etc.):
Content-type should be passed along when storing the file in swift and previously stored files need to update their mime type.
A previous case where we had to fix this is: T184871