Prevent possible XSS issues by seting text/plain for all text files in
RAW feature

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-xss' into 'master'

Fix XSS issue

When view blob as RAW make sure we allow only 2 kind of type: 'text/plain' and 'application/octet-stream'

Fixes #1514

See merge request !1045
Prevent 500 error when browse wiki page with repo access

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>