Can’t get file in Post-Controller from input in Phtml

I have a file-input field in my pthml and would like to use it to upload picture files.

[...]
                <div class="field file">
                    <label class="label" for="email"><span><?= $escaper->escapeHtmlAttr(__('Attach a picture')) ?></span></label>
                    <div class="control">
                        <input name="pictureUpload"
                               id="pictureUpload"
                               type="file"
                               class="input-file"
                               accept="image/*"
                               value="">
                    </div>
                </div>
[...]

My Post-Controller works with all other inputs but not with the file.
I tried getting it via $this->getRequest()->getFiles(); and checked $_FILES but it remains empty.
Is there another component involved that I’m missing?

My Controller extends MagentoFrameworkAppActionAction and implements MagentoFrameworkAppActionHttpPostActionInterface.