Hello, I have created my own component and use the form field in the backend:

<field
    name="file"
    type="media"
    label="COM_TOOLBOXHEKA_FIELD_FILE_LABEL"
    description="COM_TOOLBOXHEKA_FIELD_FILE_DESC"
    preview="false"
    class="span12"
    hint="text"
    accept="/image/downloads/"
    showon="type:1"
    />

In the JCE configuration I have activated both parameters in Profile > Plugin Settings > Media Field Options, so that when using the form field the JCE Media Browser is started and I can select or upload a file. But I can only select or upload files with the extension jpg, jpeg, png, apng, gif, webp, avif, although the extension pdf is entered in the JCE configuration > plugin settings > picture manager. In the global Joomla media configuration, PDF is also entered as a permitted file extension, image ending and file type. I know there is a hack where you can enter additional file types in com_media. But since this cannot be updated. I also read somewhere that the property should be added to the form field.

<field
    name="file"
    type="media"
    label="COM_TOOLBOXHEKA_FIELD_FILE_LABEL"
    description="COM_TOOLBOXHEKA_FIELD_FILE_DESC"
    preview="false"
    class="span12"
    hint="text"
    accept="/image/downloads/"
    showon="type:1"
    filter="pdf"
    />

But even then I cannot select or upload a PDF. Does anyone know a solution to this?

Hello, I was able to solve my problem! Step 1: I copied the file 'plugins / system / jce / fields / mediajce.php' to 'administrator / components / com_mycomponent / models / fields' Step 2: Change my form field in:

<field
    name="file"
    type="mediajce"
    label="COM_TOOLBOXHEKA_FIELD_FILE_LABEL"
    description="COM_TOOLBOXHEKA_FIELD_FILE_DESC"
    preview="false"
    class="span12"
    directory="/image/downloads/"
    showon="type:1"
    />

Now the media field is not an image manager but a file manager and can be set via the profile in the JCE configuration.

Sorry, I forgot to write that I changed the file 'administrator / components / com_mycomponent / models / fields / mediajce.php':

public function setup(SimpleXMLElement $element, $value, $group = null)
{
    $result = parent::setup($element, $value, $group);

    if ($result === true) {
        //$this->mediatype = isset($this->element['mediatype']) ? (string) $this->element['mediatype'] : 'images';
        $this->mediatype = "files";
     }

     return $result;
 }

This is how it works very well in my component!