bug/Extensions .mdx and .markdown not supported

[butasebi]

Describe the bug
The extensions .mdx and .markdown are being transformed to FileType.UNK when being passed to unstructured.file_utils.filetype.detect_filetype

To Reproduce
from unstructured.file_utils.filetype import detect_filetype
print(detect_filetype("file.mdx"))
print(detect_filetype("file.markdown"))

Expected behavior
The expected behavior should be either to have them go into a FileType.MDX respectively FileType.MARKDOWN (just like XLS XLSX) or at least have them be FileType.MD