The first way is after uploading your mesh, upload the texture as a decal and set the TextureID inside the meshes property as the decals id. However, if you have too many meshes, this can be painful.
I’ve experimented a bit. And found another way.
Create a textures folder inside drives root location and put your textures there. Like "C:\Textures\myTexture.png". After doing that, don’t forget to reapply the textures inside blender, but this time choose the texture inside the new textures folder that you’ve created. After that just export normally as .fbx and try importing into Roblox Studio. Your textures should be applied automatically now 
