Error reprocessing preview of revision that is different mime-type than the latest revision

The error is:

```
[2013-07-31T15:37:25.457Z] ERROR: oae-preview-processor/3773 on release0: Could not split the PDF. (contentId=c:oae:TT9x-Wj7-f, stdout="")
    Error: Command failed: Error: Unable to find file.
    Error: Failed to open PDF file: 
       /tmp/previews/c-oae-TT9x-Wj7-f/c-oae-TT9x-Wj7-f.pdf
    Errors encountered.  No output created.
    Done.  Input errors, so no output created.
    
        at ChildProcess.exithandler (child_process.js:548:15)
        at ChildProcess.EventEmitter.emit (events.js:99:17)
        at maybeClose (child_process.js:646:16)
        at Process._handle.onexit (child_process.js:688:5)
    --
    stderr: Error: Unable to find file.
    Error: Failed to open PDF file: 
       /tmp/previews/c-oae-TT9x-Wj7-f/c-oae-TT9x-Wj7-f.pdf
    Errors encountered.  No output created.
    Done.  Input errors, so no output created.
```

This was reproduced in the oae-release0 environment following today's bug bash. Steps:

1. Find a content item that was uploaded from the 0.2.0 version
2. Go into the FS and remove the previews from the original revision. This is to ensure the previews later get regenerated properly
3. Reprocess the preview by POSTing the following in the admin tenant: `$.post('/api/content/c:oae:TT9x-Wj7-f/revision/rev:oae:VVf9z-8HHQM/reprocessPreview');`
4. The above error appears in the logs and previews aren't processed