fix: check mimetype if content is present (#17456)

frappe/handler.py

@@ -206,7 +206,9 @@ def upload_file():
 	frappe.local.uploaded_file = content
 	frappe.local.uploaded_filename = filename
 
-	if not file_url and (frappe.session.user == "Guest" or (user and not user.has_desk_access())):
+	if (not file_url or content) and (
+		frappe.session.user == "Guest" or (user and not user.has_desk_access())
+	):
 		filetype = guess_type(filename)[0]
 		if filetype not in ALLOWED_MIMETYPES:
 			frappe.throw(_("You can only upload JPG, PNG, PDF, TXT or Microsoft documents."))