Keras support for .pgm files

[mohan-ys]

I am trying to use "flow_from_dataframe" for some image classification. However, the format of my files in .pgm. It looks like Keras doesn't even recognize these files(I renamed a couple of files to .jpg & they were recognized).
Is there a workaround this problem?

The error i get is "UserWarning: Found 98 invalid image filename(s) in x_col="image_path". These filename(s) will be ignored.
.format(n_invalid, x_col)"

Thank you!