add more magic bytes, use only appropriate loader when magic matches

src/common/image.h

@@ -36,16 +36,17 @@ extern "C" {
 /** return value of image io functions. */
 typedef enum dt_imageio_retval_t
 {
- DT_IMAGEIO_OK = 0, // all good :)
- DT_IMAGEIO_FILE_NOT_FOUND, // file has been lost
- DT_IMAGEIO_LOAD_FAILED, // file either corrupted or in a format not supported by the current loader,
- // and a more detailed error from among those below is not available.
+ DT_IMAGEIO_OK = 0,  // all good :)
+ DT_IMAGEIO_FILE_NOT_FOUND,  // file has been lost
+ DT_IMAGEIO_LOAD_FAILED,  // file either corrupted or in a format not supported by the current loader,
+  // and a more detailed error from among those below is not available.
  DT_IMAGEIO_UNSUPPORTED_FORMAT, // the file type is not supported; may be one which is a build-time option
  DT_IMAGEIO_UNSUPPORTED_CAMERA, // the file type is supported, but the camera model is not
  DT_IMAGEIO_UNSUPPORTED_FEATURE, // the file uses an unsupported feature such as compression type
  DT_IMAGEIO_FILE_CORRUPTED, // invalid data was detected while parsing the file
  DT_IMAGEIO_IOERROR, // a read error occurred while loading the file
- DT_IMAGEIO_CACHE_FULL // buffer allocation for image data failed
+ DT_IMAGEIO_CACHE_FULL, // buffer allocation for image data failed
+ DT_IMAGEIO_UNRECOGNIZED // file format was not recognized by loader(s)
 } dt_imageio_retval_t;
 
 typedef enum dt_imageio_write_xmp_t