[FEATURE]: Add sanity checks for file extensions for avatars (warden registration)

[SamusElderg]

Problem / opportunity:

When users created a Warden application they are able to upload undesired file formats for their avatar such as .webp

Desired outcome

Extend the checks and prevent/warn if users try to upload an undesired image format such as .webp.
Maybe for simplicity's sake, only allow png and jpg and have a max filesize check too

Another opp

We should add a sanity check for new c4 usernames and make sure they are over say 6 characters in length. The shorter names like 'pp' or '1234' can easily be duplicates and cause confusion, this would reduce that likelihood significantly