Document Active Storage implicit transformations

Author: brenogazzola

guides/source/active_storage_overview.md

@@ -850,6 +850,12 @@ location.
 <%= image_tag user.avatar.variant(resize_to_limit: [100, 100]) %>
 ```
 
+If a variant is requested, Active Storage will automatically apply 
+transformations depending on the image's format:
+
+1. Content types that are `variable` and not considered [`web images`], will be converted to PNG.
+2. If `quality` is not specified, the variant processor's default quality for the format will be used.  
+
 The default processor for Active Storage is MiniMagick, but you can also use
 [Vips][]. To switch to Vips, add the following to `config/application.rb`:
 
@@ -870,6 +876,8 @@ specific:
 ```
 
 [`variant`]: https://api.rubyonrails.org/classes/ActiveStorage/Blob/Representable.html#method-i-variant
+[`web images`]: https://github.com/rails/rails/blob/main/activestorage/lib/active_storage/engine.rb#L47
+[`variable`]: https://github.com/rails/rails/blob/main/activestorage/lib/active_storage/engine.rb#L34
 [Vips]: https://www.rubydoc.info/gems/ruby-vips/Vips/Image
 
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