Allow third-party previewers/analyzers to customize their tempdirs

activestorage/lib/active_storage/downloading.rb

@@ -3,9 +3,9 @@
 module ActiveStorage
   module Downloading
     private
-      # Opens a new tempfile and copies blob data into it. Yields the tempfile.
+      # Opens a new tempfile in #tempdir and copies blob data into it. Yields the tempfile.
       def download_blob_to_tempfile # :doc:
-        Tempfile.open("ActiveStorage") do |file|
+        Tempfile.open("ActiveStorage", tempdir) do |file|
           download_blob_to file
           yield file
         end
@@ -17,5 +17,10 @@ def download_blob_to(file) # :doc:
         blob.download { |chunk| file.write(chunk) }
         file.rewind
       end
+
+      # Returns the directory in which tempfiles should be opened. Defaults to +Dir.tmpdir+.
+      def tempdir # :doc:
+        Dir.tmpdir
+      end
   end
 end

activestorage/lib/active_storage/previewer.rb

@@ -40,8 +40,10 @@ def preview
       #       end
       #     end
       #   end
+      #
+      # The output tempfile is opened in the directory returned by ActiveStorage::Downloading#tempdir.
       def draw(*argv) # :doc:
-        Tempfile.open("ActiveStorage") do |file|
+        Tempfile.open("ActiveStorage", tempdir) do |file|
           capture(*argv, to: file)
           yield file
         end