Add more detail to what filenames are allowed in File.uri

Author: Rapptz

discord/file.py

@@ -137,8 +137,8 @@ def uri(self) -> str:
         to an uploaded file via URL.
 
         .. info::
-            Due to Discord's filename processing it can only reference files
-            that have fully ASCII filenames.
+            Due to Discord's filename processing, the filename must be ASCII aphanumeric
+            with underscores, dashes, and periods.
 
         .. versionadded:: 2.6
         """