Bug report from blog (Deskew User on July 16, 2017 at 14:37 said):
When deskewing images and converting from one format to another (for example, from jpg to tif) if the source image is not deskewed (rotation angle smaller than the default 0.01) then the source image is simply copied to the output image without conversion to the appropriate format, which results in files with wrong extension.
For example, running this command on an image that doesn’t need deskewing (can happen in batch files)
deskew -t 200 -o image_001.tif -b FFFFFF image_001.jpg
will simply copy image_001.jpg to image_001.tif WITHOUT actually converting it to TIFF format.
The workaround at the moment is to specify -l 0 or to convert every file to destination format prior to deskewing.