Add dot character `.` to legal mime subtype regular expression

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

For example, the following mimetype (used for Microsoft powerpoints) is not recognized given the current regexp : `application/vnd.ms-powerpoint; charset=binary`