[Doc] Add mime_types documentation

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+# Rspamd mime types module
+
+This module is intended to do some mime types sanity checks. That includes the following:
+
+1. Checks whether mime type is from the `good` list (e.g. `multipart/alternative` or `text/html`)
+2. Checks if a mime type is from the `bad` list (e.g. `multipart/form-data`)
+3. Checks if an attachement filename extension is different from the intended mime type
+
+## Configuration
+
+`mime_types` module reads mime types map specified in `file` option. This map contains binding
+
+```
+type/subtype score
+```
+
+When score is more than `0` then it is considered as `bad` if it is less than `0` it is considered as `good` (with the corresponding multiplier).
+When mime type is not listed then `MIME_UNKNOWN` symbol is inserted.
+
+`extension_map` option allows to specify map from a known extension to a specific mime type:
+
+~~~ucl
+extension_map = {
+  html = "text/html";
+  txt = "text/plain";
+  pdf = "application/pdf";
+}
+~~~
+
+When an attachement extension matches left part but the content type does not match the right part then symbol `MIME_BAD_ATTACHMENT` is inserted.