Reconsider export behavior when a file already exists

[manthey]

Currently, when we export a file and a file of the same name already exists, we don't replace it. In the case where it has the same size, it is likely the same file and we report that it was already copied. In the case where it has a different size, we report that we couldn't copy the file (FailedToExport appears in the report). We might want to make this behavior configurable or change it, since it leads to confusion. Possible behaviors: (a) the current behavior -- leave the original file and don't copy the new file, (b) replace the original file with the new file -- possibly dangerous, but useful if there had been a partial transfer, (c) rename one of the files.