Specifications on the names of Android resource files

Create a menu directory under the res folder under the android project, and place the configuration XML file of the menu in it.

When saving, the console keeps reporting errors. I read the error message "[09:14:44-mycontact] res \ Menu \ mainmenu. XML: invalid file name: must contain only [a-z0-9 _.] ", this is a little surprised. according to the error message, my file name is invalid. The name of the android resource file must be a combination of lowercase letters and numbers. i'm depressed. Why not use uppercase letters.

Later I changed the name of icon.png, And the Console reported an error again.

I really don't understand.