More robust check for upload files in binary mode (#2630)

* Fix check for binary mode

* Change order of type checks

---------

Co-authored-by: Tom Christie <tom@tomchristie.com>

httpx/_multipart.py

@@ -122,14 +122,14 @@ class FileField:
             # requests does the opposite (it overwrites the header with the 3rd tuple element)
             headers["Content-Type"] = content_type
 
-        if "b" not in getattr(fileobj, "mode", "b"):
-            raise TypeError(
-                "Multipart file uploads must be opened in binary mode, not text mode."
-            )
         if isinstance(fileobj, io.StringIO):
             raise TypeError(
                 "Multipart file uploads require 'io.BytesIO', not 'io.StringIO'."
             )
+        if isinstance(fileobj, io.TextIOBase):
+            raise TypeError(
+                "Multipart file uploads must be opened in binary mode, not text mode."
+            )
 
         self.filename = filename
         self.file = fileobj