Tomcat 9.0.14 and later breaks Emoticons

eh3. Issue Summary

If you upgrade tomcat to version 9.0.14 - 9.0.19, emoticons are broken. Both emoticons on pages, and the emoticon browser do not load.

Issues observed:

1. Insert Emoticon Menu
   
2. Emoticon on pages:
   
3. Global Permission page:
   
4. User icon link:
   

Environment

• Confluence 6.13.4, tomcat 9.0.14
• Confluence 6.15.3 tomcat 9.0.17

java -cp catalina.jar org.apache.catalina.util.ServerInfo
In 6.15.2 the output looks like:

Server version: Apache Tomcat/9.0.12
Server built: Sep 4 2018 22:13:41 UTC
Server number: 9.0.12.0*

Steps to Reproduce

1. Upgrade tomcat to 9.0.14 following directions here
2. Step 2
   Edit a page and load the emoticon browser and observe that the images don't load

   Expected Results

Emoticon images should load

Actual Results

emoticon images dont load

Notes

If you inspect a HAR file, you will see that in tomcat 9.0.13 and earlier, the content-type for the image is image/svg+xml;charset=UTF-8
After upgrading to tomcat 9.0.14 the content-type is text/html;charset=UTF-8

I checked the release notes for Tomcat 9.0.14, and it looks like the issue is caused by this change: "The default Servlet should not override a previously set content-type"

(Optional - If Necessary)

Workaround

java -cp catalina.jar org.apache.catalina.util.ServerInfo

We have since released Confluence 6.15.4 which contains a fix for this issue. We recommend upgrading immediately if you're impacted by the issue.

We no longer recommend the below workaround to downgrade Tomcat to 9.0.12, but it is available if required:

1. Download the Tomcat 9.0.12 lib directory Tomcat-9-0-12-lib.zip , attached to this issue (alternatively you can download and extract the [apache-tomcat-9.0.12.zip or apache-tomcat-9.0.12.tar.gz|https://archive.apache.org/dist/tomcat/tomcat-9/v9.0.12/bin/] from the Apache archives).
2. Stop Confluence
3. Back up your installation directory.
4. Remove the contents of your existing <install-directory>/lib directory.
5. Copy the contents of the downloaded lib directory to your existing <install-directory>/lib directory.
6. Restart Confluence
7. Go to General Configuration > System Information and confirm Confluence is now using Apache Tomcat 9.0.12.
8. Edit a page and confirm that you can successfully insert an emoticon.

Your browser may have cached the broken assets, so perform a hard refresh to confirm the fix has worked. If you suspect your users may also have cached broken assets, make a small change to your site colour scheme (for example change a colour and then change it back).