Add getMimeType function

FormatInterface.md

@@ -8,6 +8,7 @@ The `FormatInterface`interface defines functions that need to be implemented by
 * [getCharset](#the-getcharset-function)
 * [setExtraInfos](#the-setextrainfos-function)
 * [getExtraInfos](#the-getextrainfos-function)
+* [getMimeType](#the-getmimetype-function)
 
 Find a [template](#template) at the end of this file
 
@@ -84,6 +85,14 @@ The `getExtraInfos` function returns the information previously set via the [`se
 getExtraInfos(): array
 ```
 
+## The `getMimeType` function
+
+The `getMimeType` function returns the expected [MIME type](https://en.wikipedia.org/wiki/Media_type#Common_examples) of the format's output.
+
+```PHP
+getMimeType(): string
+```
+
 # Template
 
 This is a bare minimum template for a format: