Mail Missing Plug-In

Have created several emails and attached a jpg or pdf file. All of a sudden the jpg picture or pdf file icon goes away and I get a dashed line empth square with down arrow and above is "Missing Plug-In". What is that??
Looked on the internet and found a Mac discussion page where this topic was discussed and a fix was "Reset Safari". However, I am running macOS Sierra V10.12.5, and no where can I find "Reset Safari".
Anyone know what is going on???

Involves clearing History. Have a read of this link to fully understand clearing Safari:-


https://www.macissues.com/2015/06/22/how-to-fully-reset-safari-on-your-mac/