How to customize discourse composer editor

You don’t want to do that. See Beginner’s guide to developing Discourse Themes

If you mean “tag” in the Discourse sense, tags are for topics, not uploads or posts, so you might need to implement those some other way, or perhaps you could use the Discourse tags for your purposes.

You might rather develop a plugin, which would let you create upload-specific “tags”. You might have a look at Video Upload to YouTube and Vimeo using Theme Component for an example of something that seriously changes how uploads work. You might, for example, want to have Discourse upload those images to some other site altogether rather than make Discourse be a file manager.