Image dimensions not updated after external processing

After an image is processed with an external editor and its dimensions are altered (for example to extend a white background), C1 does not detect the new dimensions and still crops the picture to the old dimensions. "Re-generate preview" does not help. Deleting cache files does not help. Deleting cos file does not help. However, renaming the photo helps (apparently C1 thinks it's a whole new file and reads it properly only then). This seems like a bug.