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MSDN
lpstrFilter
Pointer to a buffer containing pairs of null-terminated filter strings. The last string in the buffer must be terminated by two NULL characters.
The first string in each pair is a display string that describes the filter (for example, "Text Files"), and the second string specifies the filter pattern (for example, "*.TXT"). To specify multiple filter patterns for a single display string, use a semicolon to separate the patterns (for example, "*.TXT;*.DOC;*.BAK"). A pattern string can be a combination of valid file name characters and the asterisk (*) wildcard character. Do not include spaces in the pattern string.