The plugin for saving pictures as tiff seems not be able to cope with
black/white pictures which have a transparent background. When trying to save
the picture, an error message pops up saying:

"Saving '/home/daniel/temp/tif_bug.tif' failed:
Plug-In could not save image"

Reproducible: Always

Steps to Reproduce:
- Create a new image with full colors and transparent background.
- You may save it as a file type that supports transparency, such as (I tested
these two) .png or .xcf. You may save it again at any time during this
demonstration.
- Do some painting operations in black on the picture, e.g. type some text.
- Convert the image to Black&White (Image->Mode->Indexed...->Use Black&White
(1-Bit) Palette->OK). You should now have a b/w image with some transparent
background still visible.
- Now try to save this image/export it: File -> Save As/ Save a Copy ->
[filename].tif -> A compression dialogue appears, it doesn't matter what you chose.

Actual Results:
The above-mentioned error message appears, a file [filename].tif of 8 bytes is
created.

Expected Results:
Either
a) the plugin works and saves the image correctly or
b) the user is asked whether it is ok to flatten the image for the purpose of
saving it.

Workaround:
Flatten image by hand, then saving the image as tif works alright. At least
sometimes, when I created a test image with nothing but the word "test" written
on it, I saved and reopened it. When I now tried to flatten it, it became
completely filled with black, no matter what my current background color was.

Further thoughts:
Does this bug also happen with other formats that dont support transparency?