If you refer to this question, it seems that the HTML5 spec says that this attribute is advisory (it says, that a look here).
The type attribute gives the MIME type of the linked resource. It is purely advisory. The value must be a valid MIME type.
For external resource links, the type attribute is used as a hint to user agents so that they can avoid fetching resources they do not support. If the attribute is present, then the user agent must assume that the resource is of the given type (even if that is not a valid MIME type, e.g. the empty string). If the attribute is omitted, but the external resource link type has a default type defined, then the user agent must assume that the resource is of that type. [...] If the attribute is omitted, and the external resource link type does not have a default type defined, but the user agent would obtain the resource if the type was known and supported, then the user agent should obtain the resource under the assumption that it will be supported.
So you can specify it, but it is not mandatory.