Add LIBMTP_Get_Thumbnail_Format to get thumbnail format of a file.

src/libmtp.h.in

Index: src/libmtp.h.in
diff --git a/src/libmtp.h.in b/src/libmtp.h.in
index 2b2f0bbc403ebc00a2b6aa819b85f31063a7545d..284d503a66598f502c2ef5d95d8429622e9f296b 100644
--- a/src/libmtp.h.in
+++ b/src/libmtp.h.in
@@ -924,6 +924,8 @@ int LIBMTP_Get_Representative_Sample(LIBMTP_mtpdevice_t *, uint32_t const,
                           LIBMTP_filesampledata_t *);
 int LIBMTP_Get_Thumbnail(LIBMTP_mtpdevice_t *, uint32_t const,
                          unsigned char **data, unsigned int *size);
+int LIBMTP_Get_Thumbnail_Format(LIBMTP_mtpdevice_t *device, uint32_t const id,
+                                unsigned int *format);
 
 /**
  * @}