One of the core principles of Wikipedia is "No original research". Your proposed article sounds like it was just that. The Wikipedia editors would rather have you share that research in a blog post or similar.
In other words, unless there are reliable secondary sources to base the article on, it is considered original research and is not a good fit for Wikipedia.
I guess part of the reason that new contributors feel bad is confusion about the goals and nature of Wikipedia itself.
The basic ideas of not doing original research or relying on primary sources are fine. But writing just about any article requires synthesizing multiple sources to some degree. Rules are one thing. Saying that the documentation is not a suitable source for information about a file format is something else.
The article needs independent sources (as in, secondary sources that are financially and editorially independent of the company who develops the .mix file type) to show that the topic warrants an article.
If an article has enough sources cited to show notability, primary sources like documentation pages can be used. If notability is not shown, then the topic doesn't meet Wikipedia's inclusion criteria and the content of the article is moot.
Without this requirement, any company would be able to publish promotional articles on all of its products, and exclusively use its own web pages as citations. Wikipedia's notability guidelines are in place to prevent spam and to ensure that topics only get articles if they can be written about in a neutral way.
https://en.wikipedia.org/wiki/Wikipedia:Identifying_and_usin...