Explanation
Unrecognized file formats are excluded from the search.
Certain types of files, such as Bitmap or MP3 files, don't contain content that can be indexed. As a result, the search indexing servers in Exchange and SharePoint don't perform full-text indexing on these types of files.
These types of files are considered to be unsupported file types.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/partially-indexed-items-in-content-search?view=o36
https://docs.microsoft.com/en-us/office365/securitycompliance/export-a-content-search-report