Only set the 'Conventions' attr to 'CF-1.7' for netCDF files

pygmt/datatypes/header.py

@@ -6,6 +6,7 @@
 from typing import Any, ClassVar
 
 import numpy as np
+from pygmt.enums import GridID
 
 # Constants for lengths of grid header variables.
 #
@@ -203,7 +204,14 @@ def data_attrs(self) -> dict[str, Any]:
         Attributes for the data variable from the grid header.
         """
         attrs: dict[str, Any] = {}
-        attrs["Conventions"] = "CF-1.7"
+        if self.type in {  # netCDF format
+            GridID.GMT_GRID_IS_NB,
+            GridID.GMT_GRID_IS_NS,
+            GridID.GMT_GRID_IS_NI,
+            GridID.GMT_GRID_IS_NF,
+            GridID.GMT_GRID_IS_ND,
+        }:
+            attrs["Conventions"] = "CF-1.7"
         attrs["title"] = self.title.decode()
         attrs["history"] = self.command.decode()
         attrs["description"] = self.remark.decode()