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/vnd.apple.mpegurl or perhaps application/vnd.apple.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