If you use the option to append the raw output, you can extract the mime type from there. I'm not sure what's going on in the background here, but it seems far less useful than the command line identify tool.
Imagick::identifyImage
(PECL imagick 2.0.0)
Imagick::identifyImage — Identifies an image and fetches attributes
Descrição
array Imagick::identifyImage
([ bool $appendRawOutput = false
] )
Identifies an image and returns the attributes. Attributes include the image width, height, size, and others.
Parâmetros
- appendRawOutput
-
Valor Retornado
Identifies an image and returns the attributes. Attributes include the image width, height, size, and others.
Erros
Dispara ImagickException em erro.
User Contributed Notes
Imagick::identifyImage
Imagick::identifyImage
php at ontheroad dot net dot nz
23-Nov-2010 02:04
23-Nov-2010 02:04
rob at OhReally dot nl
04-Jun-2008 10:49
04-Jun-2008 10:49
The array returned by Imagick::identifyImage():
Array
(
[imageName] => /some/path/image.jpg
[format] => JPEG (Joint Photographic Experts Group JFIF format)
[geometry] => Array
(
[width] => 90
[height] => 90
)
[type] => TrueColor
[colorSpace] => RGB
[resolution] => Array
(
[x] => 300
[y] => 300
)
[units] => PixelsPerInch
[fileSize] => 1.88672kb
[compression] => JPEG
[signature] => 9a6dc8f604f97d0d691c0286176ddf992e188f0bebba98494b2146ee2d7118da
)
Looks like the only way to get the mimetype is getimagesize()...

Imagick::hasPreviousImage