wrong mime type if the first word of a line is "extern"

### HHVM Version

HipHop VM 3.24.7
hh-07ec0d8e77593042b6de0688f9f1fba076ade535-3.24.7 Fri May 4 18:30:01 2018

### Operating System and Version

'Ubuntu 14.04'

### Standalone code, or other way to reproduce the problem

echo mime_content_type ('test_mime.srt');

[test_mime.srt.zip](https://github.com/facebook/hhvm/files/2293280/test_mime.srt.zip)

### Actual result

text/x-c

### Expected result

text/plain or application/x-subrip

If you remove "extern" from line 8 of the attached File it works as expected