Memory usage

Hi,

I just ran `bundle exec derailed bundle:mem` (given by the gem derailed_benchmarks) on a project using this gem. 
I found that :
```
TOP: 79.582 MiB
  mimemagic: 46.6289 MiB
  rails/all: 29.5742 MiB
```
  
 Mimemagic is responsible for around 58% of the memory footprint of the gems.
 
 It seems a lot to me, is anyone having this issue too ?
 
 Thanks