check for filetype using file

[bglnelissen]

the file function should be used for checking filetypes. Now we 'just' check the extension, but using file is the better way to do this. Example, now slide2Tiles did not work on .ndpi files because we allowed .tif and .svs, and did not allow .ndpi.

However:

$ file slide2.ndpi 
slide2.ndpi: TIFF image data, little-endian