What version of EE are you using? Modern (2+) use an array to identify mime type allowances. In my version I'm looking at (2.9.2), it looks like this:
$mimes = array(
'ai' => 'application/postscript',
'aif' => 'audio/x-aiff',
'aifc' => 'audio/x-aiff',
'aiff' => 'audio/x-aiff',
'avi' => 'video/x-msvideo',
'bin' => 'application/macbinary',
'bmp' => 'image/bmp',
... and so on ...
);
So you'd want to add this:
'indd' => 'application/octet-stream',
OR
'indd' => 'application/x-indesign',
OR
'indd' => 'application/x-adobe-indesign',
OR to be safe
'indd' => array('application/octet-stream',
'application/x-indesign',
'application/x-adobe-indesign'),
You'll have to try them all, the internet is reporting a few different things and it's quicker to just test these variants and see what works.
So I'm not sure what version you are running, but that appears to be a malformed mime type array (verified to be this format as far back as 2.5.5).
Edit
As Kieran McClung mentioned, EllisLab has switched to a new mimetype detection library for 2.10+. At this point, we now have a $whitelist flat array instead of a $mimes array that can have a second layer of depth. I'm pretty sure adding
'application/octet-stream'
was the fix, although Keiran said all three were needed, which would have resulted in a $whitelist array that looked like this
$whitelist = array(
'application/csv', // .csv
'application/epub+zip', // .epub
'application/excel', // .csv, .xl, .xls
'application/mac-binhex40', // .hqx
... and so on ...
'application/octet-stream', // .indd
'application/x-indesign', // .indd
'application/x-adobe-indesign', // .indd
);
And that got things whipped into shape!