I suppose but I can't be totally sure that a change made lately (PHP version 8.1
-> 8.2 or a change of hosting) is responsible of this as it worked as expected
some times before.

When selecting a file on the computer to upload via the file upload custom
field, it seems to work. Then a click on submit, then back to the event, and
there is no file attached as it should.

If I use Image attachment custom field, it's ok.

Here are the logs (but not sure it's directly related ?): 
PHP Deprecated:  Using unregistered function "trim" in a template is deprecated
and will be removed in a future release. Use Smarty::registerPlugin to
explicitly register a custom modifier. in
/public_html/lib/smarty/sysplugins/smarty_internal_compile_private_modifier.php
on line 113
PHP Warning:  Undefined array key "photoarticle" in
/public_html/tmp/templates_c/nd94^cd62ee589f55e7af07cc47f763f77e2dbb89b94b_0.cms_template.newsdetailstd.php
on line 36
PHP Warning:  Attempt to read property "value" on null in
/public_html/tmp/templates_c/nd94^cd62ee589f55e7af07cc47f763f77e2dbb89b94b_0.cms_template.newsdetailstd.php
on line 36
PHP Warning:  Undefined variable $feu_uid in
/public_html/modules/Khronos/function.displayevent.php on line 190
PHP Deprecated:  Using unregistered function "strtolower" in a template is
deprecated and will be removed in a future release. Use Smarty::registerPlugin
to explicitly register a custom modifier. in
/public_html/lib/smarty/sysplugins/smarty_internal_compile_private_modifier.php
on line 113
PHP Warning:  Undefined array key "image" in
/public_html/tmp/templates_c/985578a42ab136cd8bc62ff5cb02508bd6608451_0.cms_template.Khronoseventdetailstd.php
on line 30
PHP Warning:  Trying to access array offset on value of type null in
/public_html/tmp/templates_c/985578a42ab136cd8bc62ff5cb02508bd6608451_0.cms_template.Khronoseventdetailstd.php
on line 30
PHP Warning:  Undefined array key "image" in
/public_html/tmp/templates_c/985578a42ab136cd8bc62ff5cb02508bd6608451_0.cms_template.Khronoseventdetailstd.php
on line 35
PHP Warning:  Trying to access array offset on value of type null in
/public_html/tmp/templates_c/985578a42ab136cd8bc62ff5cb02508bd6608451_0.cms_template.Khronoseventdetailstd.php
on line 35