MIME is used to pre-code binary (8-bit) into text (7-bit) so it can be sent over text links, for example in email. Basically, the last bit is rolled in to the following character and the next two bits of that into the next character and so in the end the original binary is there but divided into smaller chunks.
TCP can carry 8-bit so if you are sending binary there is no need to do anything to encode it or decode it (unless you want to). It is only when passing the data as email you have to MIME encode it so it looks like jumbled text but the mail servers are happy. Some, but not all HTML systems also use MIME to carry images, video and music . These are all to condition the data though, the actual transfer is still done using TCP.
Brian.