Из доки:
GetFileOpenName ( title, pathname, filename[ ] {, extension {, filter { , initdir { , aFlag } } } } )

Argument Description
title A string whose value is the title of the dialog box.
pathname A string variable in which you want to store the returned path and file name.
filename, filename[ ] A string variable in which the returned file name is stored or an array of string variables in which multiple selected file names are stored. Specifying an array of string variables enables multiple selection in the dialog box.
extension (optional) A string whose value is a 1- to 3-character default file extension. The default is no extension.
filter (optional) A string whose value is a text description of the files to include in the list box and the file mask that you want to use to select the displayed files (for example, *.* or *.exe). The format for filter is:description,*. extThe default is:"All Files (*.*),*.*"
initdir (optional) A string whose value is the initial directory name. The default is the current directory.
aFlag (optional) An unsigned long whose value determines which options are enabled in the dialog box. The value of each option's flag is calculated as 2 to the power of (index -1), where index is the integer associated with the option. The value of the aggregate flag passed to GetOpenFileName is the sum of the individual option flags. See the table in the Usage section for a list of options, the index associated with each option, and the option's meaning.