fix(isDataURI): fix dataURI mediaType format

The readAsDataURL method MDN generates a dot mediaType for some file formats (.docx - application/vnd.openxmlformats-officedocument.wordprocessingml.document, . xlsx - application/vnd.openxmlformats-officedocument.spreadsheetml.sheet).
I extended the mediaType validation pattern to be compatible with readAsDataURL.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)