[ImageBam] Uploaded filenames without extension messes up key extraction

[taskhawk]

There are images that are missing an extension in the uploaded filename (I'm assuming) and that messes up the key extraction for filename and extension:

https://www.imagebam.com/image/b728aa119132443

$ gallery-dl -K "https://www.imagebam.com/image/b728aa119132443"

Keywords for directory names:
-----------------------------
category
  imagebam
extension
  34415_AlessandraAmbrosio_PhotoshootforVictoriasSecretwearingbikinislingerieonthebeachinMalibuCalifor
filename
  
image_key
  b728aa119132443
subcategory
  image
url
  https://images3.imagebam.com/d2/7a/d9/b728aa119132443.jpg

Keywords for filenames and --filter:
------------------------------------
category
  imagebam
extension
  34415_AlessandraAmbrosio_PhotoshootforVictoriasSecretwearingbikinislingerieonthebeachinMalibuCalifor
filename
  
image_key
  b728aa119132443
subcategory
  image
url
  https://images3.imagebam.com/d2/7a/d9/b728aa119132443.jpg