You're looking for the glob
module, as you said in your question, but with the wildcard on the extension:
import glob
glob.glob("mytest.*")
Example:
$ ls
a.doc a.pdf a.txt b.doc
$ python
Python 2.7.3 (default, Dec 18 2012, 13:50:09) [...]
>>> import glob
>>> glob.glob("a.*")
['a.doc', 'a.pdf', 'a.txt']
>>>