Why some jpg files wont upload

Some jpg files can upload into the slider no problem but others keep giving me an error "Unallowed File Extensions" and I cant tell the difference why it accepts some but not others. I have even resized the image to match the default images but still no success. can anyone help me?

Hi!
Perhaps, your files' are named with the use of invalid characters. The names should be written in latin letters.

Thank you I figured it out now. The jpegs that could upload had capital letter extensions

*couldn't