In T222783#5181002, @thiemowmde wrote:

We figured this is not relevant in the Wikimedia environment, because all our wikis allow the same file types. There is no way a file can show up with an extension that is not allowed on Commons.

Actually there are some files with prohibited file types, uploaded either before given file type became prohibited, or uploaded with wrong (allowed) file extension. E.g. English Wikipedia currently has 67 bmp files (media statistics). Though, I agree that this issue probably still isn't of high priority.

My suggestion for the error message: You're trying to import a file with the extension <extension name>. This file type is not allowed in this wiki.
Ideally, we would link to a list of allowed file types. Is there a default place in MediaWiki installations that defines this? (I'm thinking of a manual, i.e. something more reader-friendly than $wgFileExtensions)

@Lea_WMDE If I understand this correctly, the whole field for "new file name" makes no sense in this scenario, because I'm not able to fix this situation at all. Correct? Shouldn't we remove the field in this case?

You're trying to import a file with the extension <extension name>. This file type is not allowed in this wiki.

sounds good. I wonder if it should not be "This file extension is not allowed…" to be consistent.

I'm thinking of a manual, i.e. something more reader-friendly than $wgFileExtensions)

To understand the message above, people need to be familiar with what a file extension is, anyway, so I think the data in $wgFileExtensions is not bad (and always up-to-date). We need to render it, something like this:

The allowed extensions are: jpg, png, avi, ogg

In case the list is super long, we indeed should link somewhere.

ideally it would recognize the general type (image, sound, video) and would say: "You tried to upload a video with the extension .mp4 . This extension is not allowed on this wiki. Allowed video formats are: …"—but this is beyond MVP.

In T222783#5215389, @JStrodt_WMDE wrote:

@Lea_WMDE If I understand this correctly, the whole field for "new file name" makes no sense in this scenario, because I'm not able to fix this situation at all. Correct? Shouldn't we remove the field in this case?

This is correct, it's unrecoverable.

In T222783#5215748, @Jan_Dittrich wrote:

You're trying to import a file with the extension <extension name>. This file type is not allowed in this wiki.

sounds good. I wonder if it should not be "This file extension is not allowed…" to be consistent.

My thought behind this was that if people don't know what a file extension is, they learn that it determines the file type in the next sentence.

When I need to know the file types a wiki accepts, I typically check https://commons.wikimedia.org/wiki/Special:Upload. It currently says:

Permitted file types: tiff, tif, png, gif, jpg, jpeg, webp, xcf, mid, ogg, ogv, svg, djvu, stl, oga, flac, opus, wav, webm, mp3.

We can easily do the same and just list all extensions.

A good help page to link to is https://commons.wikimedia.org/wiki/Commons:File_types.

Note it's not easy, probably even impossible to detect a general type (e.g. video). To do this, we need to know the file extension and what it represents. But the issue we are dealing with here are unknown file extensions. We could hard-code a few known but not accepted extensions, e.g. widely used video formats we can not accept for legal reasons, and show a much nicer error message for them. But this is probably way out of scope of this extension, even for a version 2.