Return document max file size in megabytes

The same way it's done for images.

We were converting the number of megabytes to bytes and then converting
it to megabytes again. Instead, we can leave it as it is and only
convert it to bytes when necessary (only one place).

app/models/concerns/documentable.rb

@@ -12,7 +12,7 @@ module Documentable
     end
 
     def max_file_size
-      Setting["uploads.documents.max_size"].to_i.megabytes
+      Setting["uploads.documents.max_size"].to_i
     end
 
     def accepted_content_types