Add the ability to restrict image types

[ataylor32]

The "base64" and "upload" plugins both have the accept attribute set to image/*:

https://github.com/Alex-D/Trumbowyg/blob/v2.28.0/plugins/base64/trumbowyg.base64.js#L150

https://github.com/Alex-D/Trumbowyg/blob/v2.28.0/plugins/upload/trumbowyg.upload.js#L212

It would be nice if this was configurable. Currently, any image type is allowed, including .bmp, .ico, and others. I would like to have the ability to restrict this to only certain image types.