Можно использовать следующую функцию:

uses Windows, SysUtils{для функции FileExists()};

function GetLongFileName(InputName: string): string;

var

  Root, Net: Boolean;

  InPath, CurP, BegP: PChar;

  CurItem, CurPath, OutPath: string;

  RootGuard: SmallInt;

  FindHandle: Cardinal;

  FindData: WIN32_FIND_DATA;

begin

  if not FileExists(InputName) then

  begin

    Result:= '';

    Exit;

  end;{if not FileExists(InputName) then}

  OutPath:= InputName;

  InPath:= PChar(InputName);

  Root:= True;

  Net:= False;

  RootGuard:= 0;

  CurP:= InPath;

  while CurP^<>#0 do

  begin

    BegP:= CurP;

    while (CurP^<>'\') and (CurP^<>#0) do CurP := CharNext(CurP);

    SetString(CurItem, BegP, CurP - BegP);

    if CurItem='' then CurPath:= CurPath+'\' else

    begin

      CurPath:= CurPath+CurItem;

      if Root then

      begin

        OutPath:= CurPath;

        CurPath:= CurPath+'\';

      end;{if Root then}

    end;{if CurItem='' then CurPath:= CurPath+'\' else}

    if (CurPath='\\') or (CurPath='\') then Net:= True;

    if Root then

    begin

      if Net then

      begin

        RootGuard:= -1;

        Net:= False;

      end;{if Net then}

      Inc(RootGuard);

      if RootGuard>0 then Root:= False;

    end{if Root then}

    else

    begin

      FindHandle:= FindFirstFile(PChar(CurPath), FindData);

      OutPath:= OutPath+'\'+FindData.cFileName;

      Windows.FindClose(FindHandle);

      CurPath:= CurPath+'\';

    end;{if Root then ... else}

    CurP := CharNext(CurP);

  end;{while CurP^ <> #0 do}

  Result:= OutPath;

end;{GetLongFileName}