I have a channel with entries Every entry have only one field for file upload. How to add second/3-rd/4 etc.. file into entry without adding fields for new files? May be anyone know module?
I would recommend something like Matrix. You could have a single column Matrix field of file upload fields. But as for conversion of what you have? Forget it. To do what you want, you need to edit field groups, and it sounds like that isn't an option.
If it is, get Matrix, it's awesome.
Your easier option for it is to put the File field inside a Grid.
The first example on the user guide is about File fields.