Do not attempt to add metadata to a filetype that does not support it

Bloom allows the user to select an image to insert into the book and edit the copyright text for that image in one step. The copyright information is then stored in the metadata of the file, and the image is then copied the to book's directory and converted to a .png file if not already in that format. Finally, if the metadata was not successfully written before the file was copied and converted, it is written to the new .png file.

Before this pull request, if the original image file was in a format that did not support metadata, e.g. a .bmp file, and exception was thrown that resulted in the metadata not being written at all.
Thanks for catching that, it was not intentional.