How to Import 100 Private Keys

[o_e_l_e_o]

But, why would they have a private key in Base64? This is definitely not what's wrong here.

Bitaddress still gives private keys in Base64 format.

if it's a WIF it has to contain both uppercase and lowercase characters.

It can (and usually does), but it doesn't have to. 5JAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABW8UEE6 is a perfectly valid WIF key, for example.