[Laravel] Update mime types to fix the 'accept' attribute of Former::file() field

With Excel 2003 files and `mimes` live validation rules.

Otherwise Excel 2003-2007 files aren't listed in the input file dialog:
![input_file_types](https://cloud.githubusercontent.com/assets/5038872/4418500/50a778cc-4560-11e4-906b-f437878d9ea2.png)