Power BI REST API ExportTo MHTML leads to a base64 encoded file

I am using a Python Notebook to export Paginated Reports to an HTML file. Now it works fine for CSV files, I get clear and readable CSV back, the same goes for IMAGE with the format set to PNG. But when I set the format to MHTML as per the documentation I am getting a long error response about my headers, followed by a succesfull download of a file with .html extension. If I open it however it doesn't look like HTML:

And with empty formatSettings and I also left the formatSettings completely out, but there is no difference. The response from the download action for this html file is as follows:

Failed to parse headers (url=https://api.powerbi.com:443/v1.0/myorg/groups/[REDACTED]/reports/[REDACTED]/exports/Mi9CbG9iSWRWMi1mYTJjOTY5Yy00NmZlLTRiYzktYjNkZS00NjFmZTE1YmExMTN3UHY3ZGxOLXlXVXpxU3N2THlGWk5YMlN2Yzl4MnRQeUJaWk4tQ2x0Lg==/file😞 [NoBoundaryInMultipartDefect()], unparsed data: '' Traceback (most recent call last): File "/home/trusted-service-user/cluster-env/trident_env/lib/python3.11/site-packages/urllib3/connection.py", line 464, in getresponse assert_header_parsing(httplib_response.msg) File "/home/trusted-service-user/cluster-env/trident_env/lib/python3.11/site-packages/urllib3/util/response.py", line 88, in assert_header_parsing raise HeaderParsingError(defects=defects, unparsed_data=unparsed_data) urllib3.exceptions.HeaderParsingError: [NoBoundaryInMultipartDefect()], unparsed data: ''

And the file itself remains unreadable base64 encoded and it seems to be partial. Now I have a hunch that there maybe is a formatSetting to get a UTF-8 / UTF-16 HTML file, but it also seems odd that that would have to be specified. How can I find out why my HTML file is not properly formatted as readable (and usable) HTML?

Adding just ?rdl:format=MHTML to a URL of a Paginated Report returns a readable HTML file. But when using the REST API, I cannot find why it keeps sending me base64 encoded partial files.

Thanks for helping me out

Martijn Kuiper