Yes in the file it would be possible.
So I just think of the following:
You create a category in the dashboard: "not arranged"
You write down the ID (number) of it.
[CODE lang="php" title="app\themes\Peafowl\snippets\anywhere_upload.php" highlight="7"] <?php
if (get_categories()) {
?>
<div class="input-label upload-input-col center-box text-align-left">
<label for="upload-category-id"><?php _se('Category'); ?></label>
<select name="upload-category-id" id="upload-category-id" class="text-input">
<option value><?php _se('Select category'); ?></option>
<?php
foreach (get_categories() as $category) {
?>
<option value="<?php echo $category['id']; ?>"><?php echo $category['name']; ?></option>
<?php
} //for
?>
</select>
</div>
<?php
} // categories?
?>[/CODE]
Replace the line <option value><?php _se('Select category'); ?></option>
with <option value="YOURCATID"><?php _se('Select category'); ?></option>
Thus, you do not force the user to choose a category, but you have it later easier even assign the categories.
Everything else would be too complex now.