False warning about number of images per grp/page

[bertsky]

browse-ocrd/ocrd_browser/model/document.py

Lines 279 to 280 in d6ff3f3

	images = [image for image in file_index.values() if
	image.static_page_id == page_id and image.fileGrp == file_group]

This does not actually single out files with .mimetype.startswith('image/'). So we end up with a false warning:

ocrd_browser.model.document.Document.get_image_paths - Found 2 images for PAGE PHYS_0001 and fileGrp BIN, expected 1

Could also be worse than just a false warning...