diff --git a/telethon/client/uploads.py b/telethon/client/uploads.py
index 6e09b193..22e1de67 100644
--- a/telethon/client/uploads.py
+++ b/telethon/client/uploads.py
@@ -500,6 +500,11 @@ class UploadMethods:
             if pos is not None:
                 file.seek(pos)
 
+            if not isinstance(data, bytes):
+                raise TypeError(
+                    'file descriptor returned {}, not bytes (you must '
+                    'open the file in bytes mode)'.format(type(data)))
+
             file = data
             file_size = len(file)
 
diff --git a/telethon/utils.py b/telethon/utils.py
index b781e937..b2b1d89e 100644
--- a/telethon/utils.py
+++ b/telethon/utils.py
@@ -741,7 +741,7 @@ def _get_extension(file):
     elif isinstance(file, bytes):
         kind = imghdr.what(io.BytesIO(file))
         return ('.' + kind) if kind else ''
-    elif isinstance(file, io.IOBase) and file.seekable():
+    elif isinstance(file, io.IOBase) and not isinstance(file, io.TextIOBase) and file.seekable():
         kind = imghdr.what(file)
         return ('.' + kind) if kind is not None else ''
     elif getattr(file, 'name', None):