diff --git a/mastodon/Mastodon.py b/mastodon/Mastodon.py
index e6d8268..7f0f0fd 100644
--- a/mastodon/Mastodon.py
+++ b/mastodon/Mastodon.py
@@ -1722,6 +1722,8 @@ class Mastodon:
         if mime_type is None and os.path.isfile(media_file):
             mime_type = mimetypes.guess_type(media_file)[0]
             media_file = open(media_file, 'rb')
+        elif mime_type and os.path.isfile(media_file):
+            media_file = open(media_file, 'rb')
 
         if mime_type is None:
             raise MastodonIllegalArgumentError('Could not determine mime type'