uploads: add space after artist tag (fix #4392).

app/controllers/uploads_controller.rb

@@ -48,6 +48,7 @@ class UploadsController < ApplicationController
     @upload = authorize Upload.find(params[:id])
     @post = Post.new(uploader: @upload.uploader, uploader_ip_addr: @upload.uploader_ip_addr, source: @upload.source, rating: nil, **permitted_attributes(Post))
     @post.tag_string = "#{@post.tag_string} #{@upload.source_strategy&.artists.to_a.map(&:tag).map(&:name).join(" ")}".strip
+    @post.tag_string += " " if @post.tag_string.present?
 
     if request.format.html? && @upload.is_completed? && @upload.media_assets.first&.post.present?
       flash[:notice] = "Duplicate of post ##{@upload.media_assets.first.post.id}"