I do not understand German, but it looks like it is not defined because you did not add iexBitmaps to uses. iexBitmaps.FindStreamFormat
Declaration function FindStreamFormat(Stream: TStream): TIOFileType;
Description Returns the format of the file found in a stream by reading the file header. Some formats cannot be detected by content alone. Sony ARW and Kodak DCR are TIFF internally so will return as ioTIFF. WBMP and Implicit Dicom will return as ioUnknown.