Split the string ourselves

I don't know what I thought replace did, but the third argument is a count, not a start.

tubesync/sync/templatetags/filters.py

@@ -8,7 +8,7 @@ register = template.Library()
 @register.filter(is_safe=True)
 def bytesformat(input):
     output = filesizeformat(input)
-    if not (output and output.endswith('B')):
+    if not (output and output.endswith('B', -1)):
         return output
-    return output.replace('B', 'iB', -1)
+    return output[: -1 ] + output[ -1 :].replace('B', 'iB', 1)