I suspect this auto-extension voodoo has been added to MW recently. — jayvdb, IRC

Extension is considered to have a form of period and some alphanumeric characters. Each name within File: namespace should end with an extension corresponding to its file metadata type. It's possible to create a filepage with any number of pseudo extentions (File:MP_sounds.firstext.secondext.png).

When trying to upload a file with API (I use script/upload.py):

1. If filename ends with a period (.), it raises APIError: filetype-missing: The file is missing an extension.
2. If filename does not have an extension, API appends file's extension from its metadata.
3. If filename has 1 or more extensions and the last one corresponds to metadata, filename is not going to be changed.
4. If filename has 1 or more extensions and the last one does not correspond to metadata, it raises APIError: verification-error: This file did not pass file verification.

Some behaviours should be changed:

1. do not remove period, do same as 2 — to be discussed. File:MP_sounds_renamed..png is possible. It may be useful to upload files with pathetic ellipsis.
2. correct
3. correct
4. do same as 2 — to be discussed. In general it should do 2 but some “intellectual” cases may be added: say, if file's metadata type is png, filename is MP_sounds_great.jpg and MP_sounds_great.png does not exist, it may shave .jpg and append actual extension.