Fileformats.md: clarify that import of .blend files is deprecated (#5350)

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

doc/Fileformats.md

@@ -12,7 +12,6 @@ __Importers__:
 - ASE
 - ASK
 - B3D
-- [BLEND](https://en.wikipedia.org/wiki/.blend_(file_format))
 - [BVH](https://en.wikipedia.org/wiki/Biovision_Hierarchy)
 - CSM
 - COB
@@ -66,6 +65,9 @@ __Importers__:
 - XGL
 - ZGL
 
+Note: support for [BLEND](https://en.wikipedia.org/wiki/.blend_(file_format)) is deprecated.
+It is too time-consuming to maintain an undocumented format which contains so much more than we need.
+
 Additionally, some formats are supported by dependency on non-free code or external SDKs (not built by default):
 
 - [C4D](https://en.wikipedia.org/wiki/Cinema_4D) (https://github.com/assimp/assimp/wiki/Cinema4D-&-Melange) IMporting geometry + node hierarchy are currently supported