Help on Encoding .RTF File to Base 64...

Hi - Looking at encoding a .rtf file to Base64 - I've seen the Base64Encode Method and I can pass text but how would you pass a file?  Or is there another method that can do this?

The end goal is to place the encoded output in a MDM^T02 (in OBX Segment) HL7 Message.

Appreciate any pointers - Thank You