Issue Description: I cannot add custom icons.

• Dynmap Version: 3.1
• Server Version: Paper 1.16.5 build 575
• Pastebin of Configuration.txt: https://pastebin.com/V7j3RscH
• Server Host (if applicable): GameHosting (Italy)
• Pastebin of server logs: https://mclo.gs/ngW420A
• Steps to Replicate: *

1. upload my-icon.png (32x32pixel) on server root
2. run command /dmarker addicon id:my-icon my-icon file:my-icon.png (my-icon is unique name)
3. run command /dmarker icons to check if my my-icon is listed and it is listed
4. check /plugins/dynmap/web/tiles/markers/ to check if my-icon has been copied and it has been copied
5. restart server
6. run command /dmarker icons to check if my my-icon is listed and it is not listed
7. check /plugins/dynmap/web/tiles/markers/ to check if my-icon is still there and so it is

[X ] I have looked at all other issues and this is not a duplicate
[X] I have been able to replicate this

Not sure if this is still relevant but I was able to add a custom icon