It has to be in the XML property list format 1:
An URL that identifies a resource containing a string representation of a property list whose root object is a dictionary. The dictionary must contain only property list objects (instances of NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary).
See here for an overview of the property list types.
E.g. the following file:
<plist version="1.0">
<dict>
<key>A</key><string>something</string>
<key>B</key><integer>42</integer>
</dict>
</plist>
... becomes a dictionary containing:
{
A = something;
B = 42;
}