In case you're interested in the complete list of available types, here it is:
[int] - Integer
[int[]] - Integer array
[long] - Long integer
[long[]] - Long integer array
[string] - String of characters
[string[]] - String array
[char] - Single character
[char[]] - Array of characters
[bool] - True or False; [Boolean] is also valid
[bool[]] - Array of Boolean values
[byte] - Byte value
[double] - Double-precision floating number
[decimal] - Decimal number
[float] - Floating number
[single] - Single-precision floating number
[regex] - Regular expression; the .NET Framework class is System.Text.RegularExpressions.Regex
[array] - array
[xml] - XML document; the .NET Framework class is System.Xml.XmlDocument
[scriptblock] - a script block; the .NET Framework class is System.Management.Automation.ScriptBlock
[hashtable] - a hashtable or associative array; the .NET Framework class is System.Collections.Hashtable
[ref] - a PowerShell reference; the .NET Framework class is System.Management.Automation.PSReference
[psobject] - a PowerShell object; the .NET Framework class is System.Management.Automation.PSObject
[wmi] - a WMI object; the .NET Framework class is System.Management.ManagementObject
[wmisearcher] - a WMI Searcher object; the .NET Framework class is System.Management.ManagementObjectSearcher
[wmiclass] - a WMI class; the .NET Framework class is System.Management.ManagementClass