SVG Header logo is served with incorrect mimetype

Summary

When using a SVG logo as the Header image, the logo is served with the text/plain mimetype and will not display in browsers such as Chrome which enforce this. The correct mimetype should be image/svg+xml

This appears related to #55776 (moved) and #56232 (moved)

Steps to reproduce

Using 11.7 ee omnibus package upload a SVG file as the Header image and view with latest Chrome stable; the image will show as the 'broken' icon in the admin/appearance and in the header. Inspecting the request headers one observes that the file is served with an incorrect content-type header. This is a regression from 11.6.

What is the current bug behavior?

A uploaded SVG as the header logo is served with an incorrect Content-type: text/plain header

What is the expected correct behavior?

The SVG should be served with Content-type: image/svg+xml

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 16.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.5.3p105
Gem Version:    2.7.6
Bundler Version:1.16.6
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.18.1
Sidekiq Version:5.2.3
Go Version:     unknown
GitLab information
Version:        11.7.0-ee
Revision:       c02f0d4
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.8
URL:            https://gitlab.example.com
HTTP Clone URL: https://gitlab.example.com/some-group/some-project.git
SSH Clone URL:  git@gitlab.example.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers: saml
GitLab Shell
Version:        8.4.4
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 8.4.4 ? ... OK (8.4.4)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
6/1 ... yes
6/2 ... yes
6/3 ... yes
6/4 ... yes
6/5 ... yes
6/6 ... yes
6/7 ... yes
6/8 ... yes
6/9 ... yes
6/10 ... yes
6/11 ... yes
6/12 ... yes
6/13 ... yes
6/14 ... yes
6/15 ... yes
8/16 ... yes
6/17 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 5
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

The /uploads/-/system/appearance/header_logo/1/test.svg path appears to be served by gitlab-workhorse. I made a cursory look but unfortunately I do not know my way around the sources well enough to have discovered where the problem might have occurred.