To add custom emoji:

• Add the image file(s) to priv/static/emoji/custom
• In case of conflicts: add the desired shortcode with the path to config/custom_emoji.txt, comma-separated and one per line
• Force recompilation (mix clean && mix compile)

Example:

image files (in /priv/static/emoji/custom): happy.png and sad.png

content of config/custom_emoji.txt:

happy, /emoji/custom/happy.png
sad, /emoji/custom/sad.png

The files should be PNG (APNG is okay with .png for image/png Content-type) and under 50kb for compatibility with mastodon.