[Tkinter] Saving images

[Christina]

Hello,
I'm trying to create a image viewer in tkinter. I have problem with saving the images. On canvas I created an image. For save I don't use PhotoImage class but first version (photo = Image.open(somefile)). I use the following commands to save it:

 a = photo.filename = asksaveasfilename(initialdir = "/",title = "Select file",filetypes = (('JPEG', ('*.jpg','*.jpeg','*.jpe','*.jfif')),('PNG', '*.png'),('BMP', ('*.bmp','*.jdib')),('GIF', '*.gif')))
photo.save(a)

User can select what type of image he wants (jpg, png,...) but python doesn't save the type and throws out error.

Error:
ValueError: unknown file extension: 

I must write name of image like photo123.png to entry and then the image is saved. But users don't want to write .jpg,.png,... at the end of name of image.
What I have to add to the code? Some other options or function to save selected type?
Thank you, Christina