diff --git a/SPEC.md b/SPEC.md
index f162d09..b73b8e8 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -12,6 +12,7 @@ This document also uses the following terminology:
 | --- | --- |
 | SHA256(x) | An `x` value hashed via the SHA256 hashing function. |
 | base64 | The base64 data encoding format, which is described in [RFC4648](https://www.rfc-editor.org/rfc/rfc4648) |
+| MIME type | The MIME file type, as described in [RFC2045](https://www.rfc-editor.org/rfc/rfc2045) |
 
 # NAMING
 This section is more of a convention rather than a requirement.