globals.py: set instead of list for safe_file_exts

Author: NickVeld

pywebcopy/globals.py

@@ -162,7 +162,7 @@ def _cache_wrapper(self, caller, *args, **kwargs):
 ])
 """Attributes which contains multiple links."""
 
-safe_file_exts = [
+safe_file_exts = {
     '.html',
     '.php',
     '.asp',
@@ -186,7 +186,7 @@ def _cache_wrapper(self, caller, *args, **kwargs):
     '.woff',
     '.woff2',
     '.pwcf',  #: Default file extension
-]
+}
 
 safe_http_headers = {
     "Accept-Language": "en-US,en;q=0.9",