fix #685: truncate files larger than 50MB for upload

The new behavior is as follows:

1. If the file size is greater than 50MB, and the 'truncate' config
option is 'no', skip the file.

2. If the file size is greater than 50MB, and the 'truncate' config
option is 'yes', reencode the file to ogg, and truncate to first 5
minutes.