crash saving attachment

[11factory]

on v6.0.0 with a big Rails app and running a RSpec spec with file attachment, the app crash when running post_process callback.
After lot of debugging I found the issue is coming from Paperclip::MediaTypeSpoofDetector when trying to resolve the content type.
To resolve content type, paperclip is using Terrapin gem - that gem is doing a fork to run a call on hosting system but with a big rails app it crash, I think because a memory allocation issue.

As suggested by Terrapin adding posix-spawn to my Rails app solves the problem because using posix spawn use far less memory than fork.

I can do a PR with updated Readme is you think that could make sense.