When you upload a document through a Workflow Step or the Documents > Attachments menu, you can use the "File as" field to remove the file extension after selecting the file.
Once you remove the file extension, Mosaic lets you upload the file. If the file name includes a "." character, Mosaic still registers the file as that file type even if it's not listed in the allowedFileTypes system property.
For example: If you upload a file named "test.file.eml" and remove the .eml extension, Mosaic will still let you upload it even though that file type isn't allowed in the system property.
We're aware of this issue and we're working on a fix. It'll be included in Mosaic 25.1