Since 1.16 many members of this class were set to scope "protected".
The description of hook UploadForm:BeforeProcessing says:"Called just before the file data (for example description) are processed, so extensions have a chance to manipulate them."
As I have not found any way to manipulate e.g. mComment I feel this is a bug to be addressed.
(One of my extensions actively manipulated the comments till 1.15...)