how to access a List property in design view, in a custom control

 Hi there,
       I was building a custom control, extending from the FileUpload.   I needed to add a List<string> as a property (named 'FileExtensionsAllowed').  However, when it came to the design view, I kept getting a 'cannot convert from type string to List<string>' (note: error message is not word by word).  I researched a bit on it, and came upon TypeConverters, which seemed to do the job.
 
Basically, I created a type converter that could convert a string delimeted by a pipeline '|', to a list of strings.  However, when I say specified the property 'FileExtensionsAllowed' to be 'jpg|jpeg|bmp', and placed a breakpoint in the TypeConverter, it seemed to create a list of 3 strings, one per each extension.  However, when I placed a breakpoint in the page when there is the FileUpload control I created, it has a list of strings, of count 0.