Confirmed this issue exists for all field media types, so probably a PLUploader problem

The ID field in PLUploader is being cleared upon every start of a new upload, which means previously uploaded files get removed. This is not desired for multi uploads; we want to keep what we already uploaded unless we hit the clear button. This bug actually also exists in v10.

Additional PLUploader bugs also exist which need checking out. I'll make a separate issue for them.