for_collect.md

In the for_collect case, my understanding is the main goal of FilePath is to prevent clobbering files with save. If we've collected, it's now safe to overwrite our original file, so FilePath is cleared. The code has always been doing this, but the previous version tied that decision to content_type — it only cleared FilePath when content_type was also None. That coupling didn't make sense, so when I added the custom metadata field, I moved the collect-time cleanup into its own for_collect method on PipelineMetadata and made it always clear FilePath regardless of other fields.