Hi,
      Sorry, I am not sure what you meant by setting mimetype for press_release.
If you meant press_release as being a webform, then how do you associate mimetypes with webforms.
If you meant any other functionality, please explain.
Just to clarify, Below is the flow:
1:  Created Aspects and configuration settings mentioned above.
1:  Created a press_release webform using XSD
2:  During creation of a web content for the above webform, I noticed that "mimetype" was not correctly (through Debug messages).
3: So, having custom metadata extractor failed to display additional properties in the "details" page of above content.
Please correct  me know if i did wrong any thing.
Thanks