Incorrect use of explode() in setAcceptedMimeTypes

[MrJamesEllis]

https://github.com/unclecheese/silverstripe-dropzone/blob/master/code/FileAttachmentField.php#L513

2nd param to explode should be a string, should be using implode() here.

Cheerios
James

[unclecheese]

Cheers. Got that fixed.