>I don't know how to give it a valid extension.
Create a compressed folder and attach that (.zip).
Or you can rename your file to add ".txt" to the end of your .tif file and make sure you mention you did that.
Here are the valid extensions:
jpg,gif,pdf,png,doc,docx,ppt,pptx,xls,xlsx,zip,txt,sh,ksh,rar,pps,csv,log,ics,unl