All the info I found addressing this contains way too much cruft and doesn't present the actual solution clearly. Here are steps that will work, so you can get back to coding:

1. Copy the file away
2. svn revert FILENAME (in case you have local changes)
   
3. svn rm FILENAME
4. svn ci -m "Fixing checksum mismatch"
5. Copy the file back
6. svn add FILENAME
7. svn ci -m "Fixed checksum mismatch"

There you go. No more headache.