The following contains some excerpts from our docs.

The File Connector indexes user uploaded files. Currently supports .txt, .pdf, .docx, .pptx, .xlsx, .csv, .md, .mdx, .conf, .log, .json, .tsv, .xml, .yml, .yaml, .eml, and .epub files. 
You can also upload a .zip containing these files - If there are other file types in the zip, the other file types are ignored. 
There is also an optional metadata line that supports links, document owners, and time updated as metadata for Onyx’s retrieval and AI Answer.

The metadata line should be placed at the very top of the file and can take one of two formats:

#DANSWER_METADATA={"link": "<LINK>"}
<!-- DANSWER_METADATA={"link": "<LINK>"} -->
Where DANSWER_METADATA= is followed by a json. The valid json keys are:

link
primary_owners
secondary_owners
doc_updated_at
file_display_name
You can also include arbitrary key/value pairs which will be understood as “tags”. 
These tags can then be used in the UI as a filter if you want to constrain your search / conversation to only documents with certain tag(s) attached