OpenDlg->FileName возвращает полный путь
ExtractFileName( OpenDialog1->FileName ) получает имя файла
читаем хелп:
ExtractFileName
Extracts the name and extension parts of a file name.
Unit
SysUtils
Category
file name utilities
extern PACKAGE AnsiString __fastcall ExtractFileName(const AnsiString FileName);
Description
The resulting string is the rightmost characters of FileName, starting with the first character after the colon or backslash that separates the path information from the name and extension. The resulting string is equal to FileName if FileName contains no drive and directory parts.
Note:This function works for multi-byte character systems (MBCS).