Your server seems to know ico file but not pdf or xlsx file ?

Really strange … but it's a windows server ...

You can force one with magic_database in config.php but don't know how to get one for windows (i test with an old Gnu/Linux/Debian9 and a magic_database from last Gnu/Linux/Fedora version)
https://manual.limesurvey.org/Optional_settings#Upload_files

Strange maybe, but it doesn't look like adding the magic_file alone in config.php helps. I do not know how to set up the magic_database entry either. But before reaching out to sys admin for some much needed magic, I decided to wade into the source code.
After line-by-line debugging, it appears that line 181 in the source file "\limesurvey\application\controllers\UploaderController.php" (https://github.com/LimeSurvey/LimeSurvey/blob/8697d482691c59dd07b487085b42b5dfae0f4462/application/controllers/UploaderController.php#L186 of the equivalent script for version 4) generated a null string $extByMimeType.
I agree with your comment that the subsequent checks appear to suggest "Your server seems to know ico file but not pdf or xlsx file". I too thought that was odd so I carefully compared the code blocks checking the uploaded file versus checking the ico file. The difference lies in the lack of a file extension in the offending line.
It appears the magic cure I have been after is to add the file extension to the aforementioned line 181 so that it reads as follows.
$extByMimeType = LSFileHelper::getExtensionByMimeType($_FILES['uploadfile']['tmp_name']. '.' . $pathinfo['extension'], null);
Without adding the file extension, line 181 was checking a temporary file stored in C:\Windows\Temp without any file extension, leading to the null string $extByMimeType. That line without the file extension might have worked for you and others on different flavors of Unix but it appears necessary at least in IIS land.
In any event, revising line 181 to add the file extension is better than revising line 182 to turn $disableCheck to true.

Thanks so much for your time and comment, Denis. Your effort is much appreciated.

image.png (7,706 bytes)   

image.png (7,706 bytes)   

You're right …

https://github.com/LimeSurvey/LimeSurvey/blob/8697d482691c59dd07b487085b42b5dfae0f4462/application/core/LSFileHelper.php#L43 don't send null at the 3rd param

Thanks a lot for your report.

I think i add a config.php to allow disable extension checking. ONLY with php since it can be a security issue …

Fix committed to 3.x-LTS branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=30755

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=30756

@netdeals can you confirm the issue was fixed using this patch ?
if(empty(LSFileHelper::getMimeType(APPPATH."favicon.ico", null, null)) ) {

Fixed in Release 4.3.29+201130