Wand mimetypes are not right about what is supported

[lebouquetin]

Currently:

• wand tells it supports some mime types, which is wrong. Example: text/plain

Expected:

• implement a blacklist of not-really supported mime types for wand.
• allow to use substrings of mimetype in this blacklist, eg video should blacklist all video/xxx mime types

Note: when testing directly imagemagick the result is the same, so the problem comes not from wand but from imagemagick

[inkhey]

This is related to #113 , #114, #115 , #116, #117, #118