Hi Sumanth,

You can find complete list of MIME type in the link mentioned below.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Complete_list_of_MIME_types

The attachments allowed into servicenow are determined by the system property - glide.attachment.extensions - if this property is blank, that means all types of attachments are allowed in the system.

In sys_attachment we have a field as content_type which store the mime type of the attachment. Now you will have to make a comparison  of Content type with the valid mime type with the link mentioned  above. For ease purpose you can upload the valid mime type in a custom table to compare by the script in the tool itself.

Hope this helps.

Regards

Ujjawal