Problem about MediaWiki:Mwe-upwiz-tooltip-title and Wikimedia:Commons-android-strings-title info
The message doesn't say that filenames should contain spaces. It says: "You may use plain language with spaces". This means that you are allowed to use spaces if you want, but you don't have to do it.
Does this help? Or do you think that the English message should be changed?
What it means is that you should use plain readable spaces (not URL-encoded as "+" or " " or replaced by underscores) as appropriate for the normal rendering in the target orthography (for some languages, some non-ASCII spaces may be needed, including in Chinese sometimes where it could be an ideographic space, i.e. double-width, or thin spaces, and sometimes non-breaking, or zero-width spaces needed in some South-Asian scripts to separate words and avoid rendering ligatures or clusters, even if that space seems invisible but is still visible by the absence of ligation and a clear separation between the trailing or isolated form of the final letter form of the first word and the leading or isolated form of the 1st letter of the next word). As well, it may be needed to add joiner controls in some Southeastern Asian languages. In summary: use the best accurate orthography without any technical encoding. That message is still useful in Chinese because it may be displayed in the UI language of the translation tool itself, while the translation to provide is in another target language (and China has many languages from many very different families, and written using various scripts, not just Han sinograms and Han typographic rules based on fixed-width cells: even some Sinitic languages may be normally written in Latin, Cyrillic, Arabic, Tibetan scripts, or other Lolo-Burmese and Indo-Aryan scripts in their standard orthography; there are also Semitic languages, Italic languages, Germanic languages, Turkic languages, Korean; each language has its own orthographic behavior, almost all countries are multilingual even if there's a dominant language or a "normative" standard form which is still not universal; and various languages that were considered extinct, are also resurrecting with increased communication between communities spread now worldwide that want to restore their culture, and it's possible to do that now with the Internet, even in absence of support on national or regional medias).