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Your LimeSurvey version:  LimeSurvey Community Edition Version 5.3.24+220711
Own server or LimeSurvey hosting: own (Apache 2.4.64, PHP 7.4)
Survey theme/template: default fruity
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I added a file upload question to my survey and changed the max upload file size in the question settings to 10000.
In the php.ini I changed the upload_max_filesize and post_max_size to 10M.
I also added the 'max_filesize' option with '10240' to the config.
But Limesurvey ignores this all and let's me only upload file up to the size of 2048 kb.
"Sie können png, gif, doc, odt, jpg, jpeg, pdf, png, heic unter 2048 KB (pro Datei) hochladen."
Were can I change this attitude?

Best

No one ever had this?

I don't think that this is a LS issue. I would search in the PHP settings for these limits. Here it very much depends on your installation. I had cases where there was more than one place where this was set, depending on the hosting environment.

Is this a server completely setup by you? Or is it from a hosting company?

Just tested it in LS

Sorry, this file is too large. Only files upto 10240 KB are allowed.

I could upload a file with 6.978 KB, but when I tried another file with 11.170 KB I got this error message.

The 10.240 KB where set under "Maximum file size allowed (in KB)".

I doubled it to 20.480 KB and now I get this info in the file upload question:

You can upload png, gif, doc, odt, jpg, jpeg, pdf, png, heic, mp3 under 16384 KB each.

So for me this means, that somewhere in my server configuration I have a limit of 16384, just like you have a limit of 2048kb somewhere. Now you need to find where...

Found it. In my case it has been the option mysqlnd.net_cmd_buffer_size in php.ini

christianl wrote:

Found it. In my case it has been the option mysqlnd.net_cmd_buffer_size in php.ini
 

Not so sure about that. The fileupload is normally placing files into a upload folder of the survey. Why will the database setting make a difference when not involved?

Depending on your stack on the server, the bottleneck can be in webserver, php or anywhere in between.
But mysqlnd.net_cmd_buffer_size is unlikely for an unmodified LS v5. Would be surprised to see BLOBs  inside the Database of LimeSurvey.