The system has no concept of what an accidental upload is, so any upload that gets deleted will always count against your upload limit.
And just a thought here: it'd be nice if there could be a "staging area" for uploads - where you could see a preview of what your post would look like just before having it go public. I'm sure that would mostly do-away with those "ohshi- didn't mean to upload that" moments, but I do suppose triple-checking the filename on the upload page is sufficient enough.