Files with the imge/svg+xml content type can be used as XSS vectors. As such, Rails by default forces their Content-Disposition header to attachment. If you wish to use SVG images in <img> tags, you will need to remove the content type for the sanitizer.
You can use the following code in an initializer to achieve this:
# Warning: Make sure to sanitize SVGs if users gain the ability to upload themselves:
# https://github.com/rails/rails/issues/34665#issuecomment-446601748
Rails.application.config.active_storage.content_types_to_serve_as_binary.delete("image/svg+xml")
Additionally, and particularly if you intend to let users upload their own SVG content, add the active_storage_svg_sanitizer gem to your Gemfile:
gem 'active_storage_svg_sanitizer'