I suspect this is a MW core/uploading bug, rather than a site request specifically

Maybe https://noc.wikimedia.org/conf/highlight.php?file=CommonSettings.php needs a change?:

	# Dia Diagrams files --fred.
	$wgFileExtensions[] = 'dia';
	// To allow OpenOffice doc formats we need to not blacklist zip files
	$wgMimeTypeBlacklist = array_diff(
		$wgMimeTypeBlacklist,
		array( 'application/zip' ) );

application/x-gzip seems not whitelisted, only application/zip

Change 188557 had a related patch set uploaded (by Steinsplitter):
To allow dia-files (for flowcharts) we need to whitelist x-gzip

https://gerrit.wikimedia.org/r/188557

Patch-For-Review

Change 188557 merged by jenkins-bot:
To allow dia-files (for flowcharts) we need to whitelist x-gzip

https://gerrit.wikimedia.org/r/188557

In T88242#1014083, @Steinsplitter wrote:

application/x-gzip seems not whitelisted, only application/zip

This turns out not to be the case, as application/x-gzip is not blacklisted to begin with.

What's going on here is that a "compressed" dia file is really just a gzipped dia file, which is detected as "application/x-gzip" rather than "application/x-dia-diagram". This is technically correct, but not terribly helpful. It's also reminiscent of T6947.

If nothing else, you could uncompress your compressed dia files before uploading them.

So, my understanding of this is that you can upload .dia files, but if they're compressed then you hit a bug in MediaWiki? If that's correct we should close this.

In T88242#1462658, @Krenair wrote:

So, my understanding of this is that you can upload .dia files, but if they're compressed then you hit a bug in MediaWiki? If that's correct we should close this.

I can not upload dia-files, because they are compressed by the dia-program by default (like in OpenOffice). You could say, that it is normal for them to be compressed.