Description of problem:
invalid MIME type specification in wxmaxima.desktop file due to typo
and
files from wxMaxima are opened with Archive Manager by default (see additional info paragraph)
(sorry for putting two problems in one ticket; they are closely related from my point of view)

Version-Release number of selected component (if applicable):
wxMaxima.x86_64  15.04.0-1.fc22

How reproducible:
always

Steps to Reproduce:
1. noticed the problem in terminal:
$ sudo update-desktop-database
Error in file "/usr/share/applications/wxmaxima.desktop": "text-x-wxmathml" is an invalid MIME type ("text-x-wxmathml" does not contain a subtype)
...
$ grep -i "mime" "/usr/share/applications/wxmaxima.desktop"
MimeType=text-x-wxmathml;text/x-wxmaxima-batch;

2. find out correct MIME type
2.1. in terminal: wxmaxima
2.2. in wxMaxima: file -- save as... -- test.wxmx
2.3. in terminal: file test.wxmx
test.wxmx: Zip data (MIME type "text/x-wxmathml"?)
2.4. in terminal: xdg-mime query default text/x-wxmathml
# no output created 

3. correct MIME type; in "/usr/share/applications/wxmaxima.desktop" replace 
text-x-wxmathml
with
text/x-wxmathml

4. verify
$ sudo update-desktop-database
$ xdg-mime query default text/x-wxmathml
wxmaxima.desktop


Actual results:
files with MIME type 'text/x-wxmathml' are opened with Archive Manager

Expected results:
files with MIME type 'text/x-wxmathml' should be opened with wxMaxima


Additional info:
There are additional problems in the GNOME environment!
$ file test.*
test.mac:  ASCII text
test.wxm:  ASCII text
test.wxmx: Zip data (MIME type "text/x-wxmathml"?)
test.zip:  Zip archive data, at least v2.0 to extract

In nautilus:
*.mac files are opened by gedit and presented with a text icon
*.wxm files are opened by wxMaxima and presented with a wxMaxima icon
*.wxmx files are opened by Archive Manager and presented with a wxMaxima icon

In order to fix the file extension association:
## for a user
to set the default application to wxMaxima in the GNOME GUI for *.wxmx files:
nautilus -- right mouse button click on file -- properties -- open with -- wxMaxima -- [Set as default]

## system wide
delete line 6 "    <sub-class-of type="application/zip"/>" from
/usr/share/mime/packages/x-wxmathml.xml
and update MIME database:
$ sudo update-mime-database /usr/share/mime