The capable of searching contents of attachment in Custom List
We find out that sometimes the contents of files which has been attached to Custom List can be searched and sometimes cannot.
I want to know which one is correct in the specification of SharePoint Online, is the capable of searching correct or the incapable of searching correct.
Thanks for your feedback! We understand that is can be confusing. Search in SharePoint has some documented limits that have to be considered when explaining the issues that you see.
First, the supported file types: https://technet.microsoft.com/en-us/library/jj219530.aspx
Second, the limits of the parsed content of a file: https://support.office.com/en-us/article/Search-limits-for-SharePoint-Online-7c06e9ed-98b6-4304-a900-14773a8fa32f
If the files you have in the Custom List exceed these limits, or are of a different filetype that what is supported, then the content is not searchable.
I hope this explains why you are not seeing search results from these files. If your files are within the limits and of the correct filetype, please give us more information, so we can consider if this is a bug or by design.