Looking on CPAN is your best bet. If you wanted to do it yourself, I'd suggest really reading the MIME RFCs, because I can see some problems with what you have there immediately.

I'm gonna assume that the .* issues others have mentioned have been resolved, and you are using .*? for a non-greedy look-ahead. That'll help immediately.

But the end-terminator for an attachment isn't "From", it's a second line matching the first "--.*" line. Worse, the proper value of the ".*" is specified on a different line, which you don't wan't to chop out.

Ignoring that last problem, you could probably use something like:

s{
  (\n--).*?\s*\n            # Match boundary line
  Content-Type:\ image  # Find image part
  .*?                     # Match part non-greedily
  (\1)                    # Match next boundary line 
}{\1}gs                   # Replace with boundary line
[download]

This should match the proper beginnings and endings better. I'd love to get rid of the .*? parts, but I'm not sure if it can be done. I though of using \S* for the first, but the boundary line can contain spaces, so that won't work.