Q:

Attachments Search O365

I have a list with a bunch of attachments. A recent request came through to make these attachments searchable. Is the an attachment type that is searchable?

A:

Yes it is searchable however it has its own quirks. 
1) Ensure that below settings are present in your list.

2) At my end, I created a list item and uploaded a document with some unique text
3) Wait for some time for the crawl to get completed, but if you are impatient like me, just reindex the whole site collection.
4) Below is a screenshot of my list item with attachment. I have uploaded a file name test.txt and it contains the text 101.

5) Now if I search for 101 text, it will show up as below:

So it proves that list item attachments are searchable but if you take a closer look at URL, it will show the DispForm.aspx url and not the /lists/newtest/attachments/(id)/filename.  This is not user friendly and is indeed awkward for end users.
The fix for that:
Awesome blog by Mikael Svenson - Solution to display list item attachment in search result
Basically it involves creation of a custom display template where you need to use the LinkOfficeChild managed property to show the  attachment url.
LinkOfficeChild - URL of child links
Crawled properties in SharePoint
At my end, it is not created, it shows up as below:

So i suggest that you map it to one of the RefinableStringxx properties and then utilize it in your display template.

