If the website in question is publicly accessible, it would help if you'd post an example link.

If Firefox is identifying files as the wrong type, then the server is probably sending an incorrect MIME type. Use the Web Console to examine the Content-Type response header for the file request in question.

• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

Also see the following article.

• Manage file types and download actions in Firefox

With which content type does the server send these files?

• Live Http Headers: https://addons.mozilla.org/firefox/addon/live-http-headers/

• https://developer.mozilla.org/en/Tools/Web_Console

Gingerbread_Man; cor-el:

Live Headers shows this:

http://10.0.0.3/index_pop.php?action=plik_pobierz&&id=308293&disposition=inline&crc=19f7d8

GET /index_pop.php?action=plik_pobierz&&id=308293&disposition=inline&crc=19f7d8 HTTP/1.1
Host: 10.0.0.3
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20100101 Firefox/17.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: pl,en-us;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive
Referer: http://10.0.0.3/index.php?go=6&go1=6&act=62&crc=904bd4
Cookie: bs4_2_0_0=2; bs4_10_2_22=0; cookie_jezyk_uzytkownika=pl; bs4_2__0=1; bs4_1_0_2=0; bs4_6_1_12=2; bs4_10_3_25=0; calendar_1066=2; scheduler_1066=weekAndMonth; bs4_10_3_3=0; fa03676a=k90coiv75344durmah1auslfg3; sbox=1066; sfolder=1; Enabled=true

HTTP/1.1 200 OK
Date: Mon, 07 Jan 2013 11:53:32 GMT
Server: Apache/2.2.22 (Debian)
X-Powered-By: PHP/5.3.3-7+squeeze14
Expires: Mon, 26 Jul 1997 05:00:00 GMT
Cache-Control: public
Pragma: no-cache
Last-Modified: Mon, 07 Jan 2013 11:53:33 GMT
Content-Disposition: inline; filename="Ap20- praca_telefony.xlsx"
Content-Description: Ap20- praca_telefony.xlsx
Content-Length: 25907
Content-Transfer-Encoding: binary
Connection: close
Content-Type: application/zip; charset=binary

Web-console (firebug):

<div class="x-grid3-cell-inner x-grid3-col-50eab70326138" unselectable="on">
<br>
<br>
<b>Załączniki:</b>
<br>
<a class="t32" title="" href="index_pop.php?action=plik_pobierz&&id=308293&disposition=inline&crc=19f7d8" target="_blank">Ap20- praca_telefony.xlsx</a>
<a class="t32" title="" href="index_pop.php?action=plik_pobierz&&id=308293&disposition=attachment&crc=180d76" target="_blank">
25,30kB
<br>
</div>

also i have try at the begining the tip from "Change what Firefox does when you click on or download a file" and the result was that, when i change for e.g xlsx file firefox try to open zip files by excel, when i hange for zip extensions, firefox open Excel files by 7zip.

I didn't have this kind of a problem when i have used Office 2007. The problem occures with 2010 version

The Live Http response clearly shows that the server sends this file as a ZIP archive (application/zip)

• Content-Type: application/zip; charset=binary

In such a case there is not much to do about this apart from contacting the website and ask them to change the content type to one that is more appropriate or maybe better leave out that specific header as there is also a Content-Disposition header and Content-Transfer-Encoding header send.

• Content-Disposition: inline; filename="Ap20- praca_telefony.xlsx"

Content-Disposition: inline; filename="Ap20- praca_telefony.xlsx"
Content-Description: Ap20- praca_telefony.xlsx
Content-Length: 25907
Content-Transfer-Encoding: binary
Connection: close
Content-Type: application/zip; charset=binary

When writing the site owners, you can point them to the following page, which details the correct MIME types to use.

• http://technet.microsoft.com/en-us/library/cc179224.aspx

If they refuse to fix the problem, you can use the following extension to override the Content-Type. It's intended for advanced users, who know how to compose rules using regular expressions.

• https://addons.mozilla.org/firefox/addon/force-content-type/
• https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Regular_Expressions

force Content-Type it could work, but unfortunately the the files from www on which problem occures are downloaded by some script. The address of file looks like:

http://10.0.0.3/index_pop.php?action=plik_pobierz&&id=296971&disposition=inline&crc=7a9b91

and i dont know how to wrote an expression wchih will work only with xlsx files.

"/(.*?)10\.0\.0\.3(.*?)\.xlsx$/i" is not working

"/(.*?)10\.0\.0\.3(.*?)" - is chanfing for all zip files (so xlsx files are opened by Excel, but zip files also)....