Cannot upload .XLS, .XLSX, or .CSV

[d4mation]

I investigated this and it appears to be a network-wide setting in this case rather than per-site. I added the necessary file types to the Network and despite all WP Core functions telling me the File Type should be allowed, it stops it from uploading.

I’m not sure what to make of this.

(The | are present in how Core handles that Extension)

[d4mation]

With these settings, the following returns that it should allow the file to be uploaded:

var_dump( check_upload_mimes( array( 'xlsx' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet' ) ) );

However, it is stopped.