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

[scanny]

@butasebi I don't think the .mdx is going to work out because that format cannot be parsed by the python-markdown package we use. It's really a hybrid format.

But a file with .markdown should identify as such. I'm currently getting FileType.TXT for that case, where the same file with a .md extension is correctly identified. I'll have a closer look at why that is.

[scanny]

Yep, okay, found that one. I'll get that into the next release.

[scanny]

Fixed in #3828.