Add docs on how to configure Active Storage

[ci skip]

guides/source/configuring.md

@@ -752,6 +752,41 @@ main application.
 You can set this as nil to not mount Action Cable as part of your
 normal Rails server.
 
+
+### Configuring Active Storage
+
+`config.active_storage` provides the following configuration options:
+
+* `config.active_storage.analyzers` accepts an array of classes indicating the analyzers available for Active Storage blobs. The default is `[ActiveStorage::Analyzer::ImageAnalyzer, ActiveStorage::Analyzer::VideoAnalyzer]`. The former can extract width and height of an image blob; the latter can extract width, height, duration, angle, and aspect ratio of a video blob.
+
+* `config.active_storage.previewers` accepts an array of classes indicating the image previewers available in Active Storage blobs. The default is `[ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer]`. The former can generate a thumbnail from the first page of a PDF blob; the latter from the relevant frame of a video blob.
+
+* `config.active_storage.paths` accepts a hash of options indicating the locations of previewer/analyzer commands. The default is `{}`, meaning the commands will be looked for in the default path. Can include any of these options:
+    * `:ffprobe` - The location of the ffprobe executable.
+    * `:mutool` - The location of the mutool executable.
+    * `:ffmpeg` - The location of the ffmpeg executable.
+
+   ```ruby
+   config.active_storage.paths[:ffprobe] = '/usr/local/bin/ffprobe'
+   ```
+
+* `config.active_storage.variable_content_types` accepts an array of strings indicating the content types that Active Storage can transform through ImageMagick. The default is `%w(image/png image/gif image/jpg image/jpeg image/vnd.adobe.photoshop)`.
+
+* `config.active_storage.content_types_to_serve_as_binary` accepts an array of strings indicating the content types that Active Storage will always serve as an attachment, rather than inline. The default is `%w(text/html
+text/javascript image/svg+xml application/postscript application/x-shockwave-flash text/xml application/xml application/xhtml+xml)`.
+
+* `config.active_storage.queue` can be used to set the name of the Active Job queue used to perform jobs like analyzing the content of a blob or purging a blog.
+
+  ```ruby
+  config.active_job.queue = :low_priority
+  ```
+
+* `config.active_storage.logger` can be used to set the logger used by Active Storage. Accepts a logger conforming to the interface of Log4r or the default Ruby Logger class.
+
+  ```ruby
+  config.active_job.logger = ActiveSupport::Logger.new(STDOUT)
+  ```
+
 ### Configuring a Database
 
 Just about every Rails application will interact with a database. You can connect to the database by setting an environment variable `ENV['DATABASE_URL']` or by using a configuration file called `config/database.yml`.