feat: support .md and .mdx file uploads

Plain-text files have no magic bytes, so the file-type signature
detector returned an empty MIME type and the backend rejected them.
Add an extension-based fallback for markdown/mdx, allow text/mdx in
the API and editor allow-lists, and pick a file icon for these
extensions.

apps/api/plane/settings/common.py

@@ -402,6 +402,7 @@ ATTACHMENT_MIME_TYPES = [
     "application/vnd.openxmlformats-officedocument.presentationml.presentation",
     "text/plain",
     "text/markdown",
+    "text/mdx",
     "application/rtf",
     "application/vnd.oasis.opendocument.spreadsheet",
     "application/vnd.oasis.opendocument.text",
@@ -469,8 +470,6 @@ ATTACHMENT_MIME_TYPES = [
     "application/x-sql",
     # Gzip
     "application/x-gzip",
-    # Markdown
-    "text/markdown",
 ]
 
 # Seed directory path

apps/web/core/components/icons/attachment/attachment-icon.tsx

@@ -47,6 +47,9 @@ export const getFileIcon = (fileType: string, size: number = 28) => {
     case "js":
       return <JavaScriptIcon height={size} width={size} />;
     case "txt":
+    case "md":
+    case "markdown":
+    case "mdx":
       return <TxtIcon height={size} width={size} />;
     case "svg":
       return <SvgIcon height={size} width={size} />;

packages/editor/src/core/constants/config.ts

@@ -33,6 +33,7 @@ export const ACCEPTED_ATTACHMENT_MIME_TYPES = [
   "application/vnd.openxmlformats-officedocument.presentationml.presentation",
   "text/plain",
   "text/markdown",
+  "text/mdx",
   "application/rtf",
   "audio/mpeg",
   "audio/wav",

packages/services/src/file/helper.ts

@@ -10,6 +10,28 @@ import { fileTypeFromBuffer } from "file-type";
 import type { TFileMetaDataLite, TFileSignedURLResponse } from "@plane/types";
 import { DANGEROUS_EXTENSIONS } from "@plane/constants";
 
+/**
+ * @description Map of file extensions to MIME types for plain-text formats that
+ * file-type signature detection cannot identify (no magic bytes).
+ */
+const EXTENSION_MIME_TYPE_MAP: Record<string, string> = {
+  md: "text/markdown",
+  markdown: "text/markdown",
+  mdx: "text/mdx",
+};
+
+/**
+ * @description Resolve a MIME type from the file extension for known text formats.
+ * @param {string} filename
+ * @returns {string} MIME type if extension is known, empty string otherwise
+ */
+const detectMimeTypeFromExtension = (filename: string): string => {
+  const parts = filename.split(".");
+  if (parts.length < 2) return "";
+  const extension = parts[parts.length - 1]?.toLowerCase() || "";
+  return EXTENSION_MIME_TYPE_MAP[extension] || "";
+};
+
 /**
  * @description Filename validation - checks for double extensions and dangerous patterns
  * @param {string} filename
@@ -103,6 +125,12 @@ const validateAndDetectFileType = async (file: File): Promise<string> => {
     console.warn("Error detecting file type from signature:", _error);
   }
 
+  // Plain-text formats (markdown, mdx, …) have no magic bytes — fall back to extension.
+  const extensionType = detectMimeTypeFromExtension(file.name);
+  if (extensionType) {
+    return extensionType;
+  }
+
   // fallback for unknown files
   return "";
 };