As far as I know there is no required format for WMTS services, all you can do is request the getcapabilities document and then for each layer there are one or more <Format> blocks that you should look for to tell you what formats are available for that layer:
<Layer>
<ows:Title>OS OpenData Raster (greyscale)</ows:Title>
<ows:WGS84BoundingBox>
<ows:LowerCorner>-179.8425860941661 -89.92400664174129</ows:LowerCorner>
<ows:UpperCorner>179.97997214501078 89.99264625256251</ows:UpperCorner>
</ows:WGS84BoundingBox>
<ows:Identifier>sf:osopenrasterbw</ows:Identifier>
<Style isDefault="true">
<ows:Identifier/>
</Style>
<Format>image/png</Format>
<Format>image/jpeg</Format>