when we get a SynapseMd5MismatchError we should delete the downloaded file
For local files (ExternalFileHandle), md5 mismatch should not delete the file.
For http/ftp downloads, the downloaded file should be deleted if expected md5 does not match
Reopening because we have decided to remove deletion of mismatching files. We must consider all edge cases so that we do no accidentally delete user's local files. One such case was if the user supplied MD5 did not match that of locally stored file (ExternalFileHandle externalUrl=file://...)