diff --git a/mastodon/Mastodon.py b/mastodon/Mastodon.py
index a5c05b7..a0cf907 100644
--- a/mastodon/Mastodon.py
+++ b/mastodon/Mastodon.py
@@ -1495,7 +1495,7 @@ class Mastodon:
         
         # Load avatar, if specified
         if not avatar is None:
-            if avatar_mime_type is None and os.path.isfile(avatar):
+            if avatar_mime_type is None and (isinstance(avatar, str) and os.path.isfile(avatar)):
                 avatar_mime_type = guess_type(avatar)
                 avatar = open(avatar, 'rb')
 
@@ -1504,7 +1504,7 @@ class Mastodon:
         
         # Load header, if specified
         if not header is None:
-            if header_mime_type is None and os.path.isfile(header):
+            if header_mime_type is None and (isinstance(avatar, str) and os.path.isfile(header)):
                 header_mime_type = guess_type(header)
                 header = open(header, 'rb')
 
@@ -1724,10 +1724,10 @@ class Mastodon:
         Returns a `media dict`_. This contains the id that can be used in
         status_post to attach the media file to a toot.
         """
-        if mime_type is None and os.path.isfile(media_file):
+        if mime_type is None and (isinstance(media_file, str) and os.path.isfile(media_file)):
             mime_type = guess_type(media_file)
             media_file = open(media_file, 'rb')
-        elif mime_type and os.path.isfile(media_file):
+        elif isinstance(media_file, str) and os.path.isfile(media_file):
             media_file = open(media_file, 'rb')
 
         if mime_type is None: