letter or $8\mathrm{.}14.$ A computer operating system allows files to be named using any combination of uppercase letters $(A-Z)$ and digits $(0-9),$ but the number of characters in the file name is at most eight $($and there has to be at least one character in the file name$).$ For example, $x23,W,4AA,$ and ABCD$1234$ are valid file names, but $W-23$ and WONDERFUL are not valid $($the first has an improper character, and the second is too long$).$

How many different file names are possible in this system?