I re-checked it, the file size is 904457, that's right, and yes, the reported error position is 904465, which is greater than 904457.
Here's the complete set of files of my last test (note: fix the absolute path in test script before trying it)
box.net/shared/qb3byphc6u
Result:
MIME message decoding error: incomplete message body part at position 904465 line 12249 column 1
Is is possible to have incorrect CR/LF conversions during file reading ?