How did you format your Windows partition during the Boot Camp install - NTFS? If you did, you're not going to be able to work with those files. If you can get by with a 32 GB or less Windows partition, formatting the partition in FAT will allow read/write access. This is all in the Apple Boot Camp installation guide.
If you choose to reformat, you have to delete the Boot Camp partition and reinstall, then choose FAT during the new setup. Goes without saying you'll need to back everything up.
If you need a bigger partition than 32 GB, I think there are third party software solutions for NTFS - Paragon software I think has one but now you're passing my knowledge point ...
Cheers