This answer is one of various possible solutions for a "Bash function that accepts input from parameter or pipe" since the OP indicated [in a comment] that base64 was not the actual problem domain. As such, it makes no attempt to assure input is able to be directly read from a file.

I don't like the first syntax because then you have weird looking code due to all the leading whitespace, and you lose a lot of real estate.

Any fool can write code that a computer can understand. Good programmers write code that humans can understand.

Code explains what and how Documentation explains why.