Hi Raj,
Please refer to the answers of the two experts above.
If you need to upload a file to D365 as an attachment, then you need to set the size limit of the attachment first.
Go to System Settings, under Email tab, we can see Maximum file size (in kilobytes), it defaults to 5,120 (5mb).

We can modify it to the maximum of 131,072 (128mb).

In this case, the single file you upload needs to be less than 128mb.