File upload: docx files are interpreted as zip

[bspekker]

Describe the bug
Sometimes the upload of a docx file leads to the following error message, indicating that the file is interpreted as a zip:

Fehler: FILENAME.docx does not have a file extension that matches the determined mimetype application/zip.

How to Reproduce

1. contact me to get the example file
2. open a meeting, go to files
3. upload the file

Expected behavior
Ensure that docx files (and other allowed types) can be uploaded.

For example check if OpenSlides might take any other meta information from the file to determine the filetype that is not the file extension.

[luisa-beerboom]

Quick google search tells me that docx files are basically zip containers with multiple XML documents within, therefore the content checker identifies them as such.

Same goes for .pptx and .xlsx files btw.

The easiest solution for this would probably be to just whitelist these files for the application/zip mimetype since that's what they're gonna read them as.

Additionally, if we have security concerns f.E., we could try to look into our mimetype checking library and possibly replacing or updating it, or try to check the content manually by trying to unzip it.