Fix relocate.mime_type if slashes in subtype (#11788)

If the mimetype returned from `file -h -b --mime-type` contains slashes
in its subtype, the tuple returned from `spack.relocate.mime_type` will
have a size larger than two, which leads to errors.

Change-Id: I31de477e69f114ffdc9ae122d00c573f5f749dbb

lib/spack/spack/relocate.py

@@ -749,4 +749,5 @@ def mime_type(file):
     tty.debug('[MIME_TYPE] {0} -> {1}'.format(file, output.strip()))
     if '/' not in output:
         output += '/'
-    return tuple(output.strip().split('/'))
+    split_by_slash = output.strip().split('/')
+    return (split_by_slash[0], "/".join(split_by_slash[1:]))