Ez a konkrét eset a weboldal hibájából fakad, ugyanis a webkiszolgáló nem tölti ki a „content-type” HTTP-fejlécet, amely a fájl MIME-típusát tartalmazná (XLSX esetén egyébként application/vnd.openxmlformats-officedocument.spreadsheetml.sheet értéket kellene tartalmaznia).

In English: The current issue is caused by the website itself, the web server does not fill the „content-type” HTTP header, which would contain the MIME type of the file (for XLSX the correct MIME type would be application/vnd.openxmlformats-officedocument.spreadsheetml.sheet).

Unfortunately, this did not solve the problem. Content-Type is optional and this error has not occurred before. I tried, but nothing changed.

What have you tried exactly? Content-Type is indeed not mandatory, but it SHOULD be set (all caps, meaning the RFC says so). Even Chromium complains that the Content-Type is invalid, it just detects the actual MIME type through MIME sniffing.

Thank you for your help. I placed in the head the next: <meta http-equiv="content-type" content="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"> For this nothing changed. Firefox on my iPhon can open this Excel file correctly.

First of all: Firefox for iOS is actually a skinned Safari, so it behaves differently than "normal" (Gecko-based) Firefox.

That meta tag is not valid, and it also does not make much sense. You don't need to set Content-Type at a HTML file, but in the HTTP headers. So to fix this issue, I'd need to know what web server do you use.

My webpage is on Hostinger server.

Then I guess you cannot edit your webserver configuration, so you should contact their support with your issue.

Thanks for the help! I finally found the solution on the internet. I enclose the two links that can help others if they run into this problem: https://developer.mozilla.org/en-US/docs/Learn/Server-side/Configuring_server_MIME_types https://htaccessbook.com/useful-htaccess-rules/