Minecraft v1.2.0.18 Beta won't accept certain skins

^{Update by} @unknown^:
Summary:
Skins that have the file extension in uppercase will not open.

Steps to Reproduce:

1. Try and equip a skin that has .PNG file extension

Observed Results:
The skin will not be accepted, and the message "That's not a minecraft skin, silly." shows.

Expected Results:
I expected the skin to work even if the file extension is in uppercase.

Screenshots/Videos attached: Yes

Notes:
Changing the skin file to lowercase enables it to be recognised and used.

Original Description:
After the newest build i am able to use skins and they won't appear black. However the game won't allow me to use certain skins, most of them i noticed are .PNG files as apposed to .png. If i use .png then it will work fine and if i use .PNG it will say "That's not a minecraft skin, silly." I don't know why it really matters if it's capitalized or not. Sorry in advance if this is a duplicate but the internet won't help me with this one.