Looks like it is a regular expression issue.  The code is trying to
delete the last linend before the boundary, which belongs to the
boundary according to the RFC, but it does so with the following RE:

    (\r\n|\r|\n)$

This RE matches '\r\n' in '\r\n\n', which is what Guido's message had. 
The code then deletes the number of characters equal to the length of
the match.  So yes, it is a mixed line ending problem.