3 files changed, 20 insertions, 6 deletions
diff --git a/activestorage/app/models/active_storage/blob.rb b/activestorage/app/models/active_storage/blob.rb
index acaf22fac1..fdf17ac67e 100644
--- a/activestorage/app/models/active_storage/blob.rb
+++ b/activestorage/app/models/active_storage/blob.rb
@@ -16,6 +16,7 @@ require "active_storage/analyzer/null_analyzer"
 # update a blob's metadata on a subsequent pass, but you should not update the key or change the uploaded file.
 # If you need to create a derivative or otherwise change the blob, simply create a new blob and purge the old one.
 class ActiveStorage::Blob < ActiveRecord::Base
+  class InvariableError < StandardError; end
   class UnpreviewableError < StandardError; end
   class UnrepresentableError < StandardError; end
 
@@ -110,6 +111,7 @@ class ActiveStorage::Blob < ActiveRecord::Base
     content_type.start_with?("text")
   end
 
+
   # Returns an ActiveStorage::Variant instance with the set of +transformations+ provided. This is only relevant for image
   # files, and it allows any image to be transformed for size, colors, and the like. Example:
   #
@@ -125,8 +127,20 @@ class ActiveStorage::Blob < ActiveRecord::Base
   #
   # This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController
   # can then produce on-demand.
+  #
+  # Raises ActiveStorage::Blob::InvariableError if ImageMagick cannot transform the blob. To determine whether a blob is
+  # variable, call ActiveStorage::Blob#previewable?.
   def variant(transformations)
-    ActiveStorage::Variant.new(self, ActiveStorage::Variation.wrap(transformations))
+    if variable?
+      ActiveStorage::Variant.new(self, ActiveStorage::Variation.wrap(transformations))
+    else
+      raise InvariableError
+    end
+  end
+
+  # Returns true if ImageMagick can transform the blob (its content type is in +ActiveStorage.variable_content_types+).
+  def variable?
+    ActiveStorage.variable_content_types.include?(content_type)
   end
 
 
@@ -170,16 +184,16 @@ class ActiveStorage::Blob < ActiveRecord::Base
     case
     when previewable?
       preview transformations
-    when image?
+    when variable?
       variant transformations
     else
       raise UnrepresentableError
     end
   end
 
-  # Returns true if the blob is an image or is previewable.
+  # Returns true if the blob is variable or is previewable.
   def representable?
-    image? || previewable?
+    variable? || previewable?
   end
 
 
diff --git a/activestorage/app/models/active_storage/filename.rb b/activestorage/app/models/active_storage/filename.rb
index 79d55dc889..b9413dec95 100644
--- a/activestorage/app/models/active_storage/filename.rb
+++ b/activestorage/app/models/active_storage/filename.rb
@@ -50,7 +50,7 @@ class ActiveStorage::Filename
     @filename.encode(Encoding::UTF_8, invalid: :replace, undef: :replace, replace: "�").strip.tr("\u{202E}%$|:;/\t\r\n\\", "-")
   end
 
-  def parameters
+  def parameters #:nodoc:
     Parameters.new self
   end
 
diff --git a/activestorage/app/models/active_storage/filename/parameters.rb b/activestorage/app/models/active_storage/filename/parameters.rb
index 58ce198d38..fb9ea10e49 100644
--- a/activestorage/app/models/active_storage/filename/parameters.rb
+++ b/activestorage/app/models/active_storage/filename/parameters.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class ActiveStorage::Filename::Parameters
+class ActiveStorage::Filename::Parameters #:nodoc:
   attr_reader :filename
 
   def initialize(filename)