Feature: use file type limitation during selection

[Baloobear]

Hi,

although the plugin complains if an unsupported file type is tried to upload it would be more convenient if the user is already limited during the selection of the files.

This could be realized with the accept attribute of the input field, e.g. <input type="file" multiple accept=".png, .jpg, .jpeg, .gif" id="xyz" name="xyz" />.