Ciao a tutti.
Visto la grande partecipazione al topic mi rispondo da solo 
Ho risolto, Googlando un po' di piu' e chiedendo su IRC.
Il problema era mod_deflate che comprimeva anche gli zip e gli mp3 e questo non permetteva al browser di capire la dimensione totale dei files.
Ho seguito alcuni indicazioni lette qui: http://www.geekride.com/apache-enabl...site-optimise/
Ho modificato httpd.conf da cosi:
codice:
<IfModule mod_deflate.c>
<Location />
# Insert filter
SetOutputFilter DEFLATE
# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine
# BrowserMatch \bMSIE no-gzip !gzip-only-text/html
# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch \bMSI[E] no-gzip !gzip-only-text/html
# Don't compress images
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</Location>
</IfModule>
A cosi:
codice:
<IfModule mod_deflate.c>
<Location />
# Insert filter
SetOutputFilter DEFLATE
# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine
# BrowserMatch \bMSIE no-gzip !gzip-only-text/html
# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch \bMSI[E] no-gzip !gzip-only-text/html
# .CUSTOM.
# Configuro la non compressione per immagini, zip, file gia' compressi ed mp3.
# Originale era: \.(?:gif|jpe?g|png)$ no-gzip dont-vary
#
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png|swf|pdf|exe|t?gz|zip|bz2|sit|rar|mp3)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</Location>
</IfModule>
In pratica cambiando questo:
codice:
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png)$ no-gzip dont-vary
in questo:
codice:
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png|swf|pdf|exe|t?gz|zip|bz2|sit|rar|mp3)$ no-gzip dont-vary
Spero possa essere utile a qualcun altro.
Ciao a tutti!